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.
Merge pull request #27 from StarvingMarvin/master

Added several more extensions and filenames into :exts