Drop old ruby versions which are no longer tested

[nobu]

It doesn't seem working with ruby 2.1 already.

$ ruby2.1 -I./lib exe/rake 
/opt/local/bin/ruby2.1 -w -I"lib:test"  "/Users/nobu/src/ruby/rake/lib/rake/rake_test_loader.rb" "test/test_private_reader.rb" "test/test_rake.rb" "test/test_rake_application.rb" "test/test_rake_application_options.rb" "test/test_rake_backtrace.rb" "test/test_rake_clean.rb" "test/test_rake_cpu_counter.rb" "test/test_rake_definitions.rb" "test/test_rake_directory_task.rb" "test/test_rake_dsl.rb" "test/test_rake_early_time.rb" "test/test_rake_extension.rb" "test/test_rake_file_creation_task.rb" "test/test_rake_file_list.rb" "test/test_rake_file_list_path_map.rb" "test/test_rake_file_task.rb" "test/test_rake_file_utils.rb" "test/test_rake_functional.rb" "test/test_rake_invocation_chain.rb" "test/test_rake_late_time.rb" "test/test_rake_linked_list.rb" "test/test_rake_makefile_loader.rb" "test/test_rake_multi_task.rb" "test/test_rake_name_space.rb" "test/test_rake_package_task.rb" "test/test_rake_path_map.rb" "test/test_rake_path_map_explode.rb" "test/test_rake_path_map_partial.rb" "test/test_rake_pseudo_status.rb" "test/test_rake_rake_test_loader.rb" "test/test_rake_reduce_compat.rb" "test/test_rake_require.rb" "test/test_rake_rules.rb" "test/test_rake_scope.rb" "test/test_rake_task.rb" "test/test_rake_task_argument_parsing.rb" "test/test_rake_task_arguments.rb" "test/test_rake_task_manager.rb" "test/test_rake_task_manager_argument_resolution.rb" "test/test_rake_task_with_arguments.rb" "test/test_rake_test_task.rb" "test/test_rake_thread_pool.rb" "test/test_rake_top_level_functions.rb" "test/test_rake_win32.rb" "test/test_thread_history_display.rb" "test/test_trace_output.rb" 
/opt/local/lib/ruby2.1/2.1.0/rubygems/dependency.rb:298:in `to_specs': Could not find 'minitest' (~> 5) - did find: [minitest-4.7.5] (Gem::LoadError)
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/dependency.rb:309:in `to_spec'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_gem.rb:53:in `gem'
	from /Users/nobu/src/ruby/rake/test/helper.rb:13:in `<top (required)>'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `require'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `rescue in require'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_require.rb:144:in `require'
	from /Users/nobu/src/ruby/rake/test/test_private_reader.rb:2:in `<top (required)>'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `require'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_require.rb:135:in `rescue in require'
	from /opt/local/lib/ruby2.1/2.1.0/rubygems/core_ext/kernel_require.rb:144:in `require'
	from /Users/nobu/src/ruby/rake/lib/rake/rake_test_loader.rb:17:in `block in <main>'
	from /Users/nobu/src/ruby/rake/lib/rake/rake_test_loader.rb:5:in `select'
	from /Users/nobu/src/ruby/rake/lib/rake/rake_test_loader.rb:5:in `<main>'
rake aborted!
Command failed with status (1): [ruby -w -I"lib:test"  "/Users/nobu/src/ruby/rake/lib/rake/rake_test_loader.rb" "test/test_private_reader.rb" "test/test_rake.rb" "test/test_rake_application.rb" "test/test_rake_application_options.rb" "test/test_rake_backtrace.rb" "test/test_rake_clean.rb" "test/test_rake_cpu_counter.rb" "test/test_rake_definitions.rb" "test/test_rake_directory_task.rb" "test/test_rake_dsl.rb" "test/test_rake_early_time.rb" "test/test_rake_extension.rb" "test/test_rake_file_creation_task.rb" "test/test_rake_file_list.rb" "test/test_rake_file_list_path_map.rb" "test/test_rake_file_task.rb" "test/test_rake_file_utils.rb" "test/test_rake_functional.rb" "test/test_rake_invocation_chain.rb" "test/test_rake_late_time.rb" "test/test_rake_linked_list.rb" "test/test_rake_makefile_loader.rb" "test/test_rake_multi_task.rb" "test/test_rake_name_space.rb" "test/test_rake_package_task.rb" "test/test_rake_path_map.rb" "test/test_rake_path_map_explode.rb" "test/test_rake_path_map_partial.rb" "test/test_rake_pseudo_status.rb" "test/test_rake_rake_test_loader.rb" "test/test_rake_reduce_compat.rb" "test/test_rake_require.rb" "test/test_rake_rules.rb" "test/test_rake_scope.rb" "test/test_rake_task.rb" "test/test_rake_task_argument_parsing.rb" "test/test_rake_task_arguments.rb" "test/test_rake_task_manager.rb" "test/test_rake_task_manager_argument_resolution.rb" "test/test_rake_task_with_arguments.rb" "test/test_rake_test_task.rb" "test/test_rake_thread_pool.rb" "test/test_rake_top_level_functions.rb" "test/test_rake_win32.rb" "test/test_thread_history_display.rb" "test/test_trace_output.rb" ]
exe/rake:27:in `<main>'
Tasks: TOP => default => test
(See full trace by running task with --trace)