Omit spec files when building gem

[dduugg]

Description

This reduces the gem size by ~20% (902k->738k) by omitting spec files (there may be further space optimizations in this style, this PR doesn't attempt to be comprehensive). This is inspired by the default implementation when creating a gem via bundle gem, which is:

`git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }

I've also cleaned up .rubocop.yml to resolve local plugin errors. (Let me know if I've overstepped the PR scope and i'll cut it.)

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[coveralls]

Coverage decreased (-0.08%) to 93.427% when pulling d4e05ed on dduugg:omit-spec-files into 872b3ac on lsegal:master.

[lsegal]

Thanks for the fix!