Do not include `/spec` in production gem

[tagliala]

https://rubygems.org/gems/money-rails

The gem is now 50K

Without spec files, it can be reduced to 28K

Also, test_files is softly deprecated as per rubygems/guides#90 and flagged as an offense by RuboCop: https://docs.rubocop.org/rubocop/cops_gemspec.html#gemspecdeprecatedattributeassignment