Added several more extensions and filenames into :exts

Specifically, added :gemspec, :Rakefile and :Gemfile
Last two won't work on current LightTable release (0.6.2) but won't
break anything either. LightTable supports entire filenames as type
patterns as of commit [b5ec8d7](https://github.com/LightTable/LightTable/commit/b5ec8d7e0bacbc6b99f269e90c16ead366f6cdf6).