Specify license in gem metadata

[musicinmybrain]

This is good practice, and fixes the following warning:

$ gem build ast-tdl.gemspec
WARNING:  licenses is empty, but is recommended.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
WARNING:  See https://guides.rubygems.org/specification-reference/ for help

[firefly-cpp]

Good point. Thanks @musicinmybrain!