Require "date"

Seems to be required by oj, but there's no oj for JRuby so the date
benchmarks fail if I remove it.

Author: headius

benchmark/encoder.rb

@@ -1,5 +1,6 @@
 require "benchmark/ips"
 require "json"
+require "date"
 require "oj"
 
 Oj.default_options = Oj.default_options.merge(mode: :compat)