fix the add_note_to_loginscreen recipe

The plist needs to be under /Library/Preferences, not the default location
clean up .travis.yml a little

* declare the language explicitly
* put `gem install foodcritic` into the install step
Merge pull request #15 from bbeck/standard_function_keys

Add recipe for enabling standard function keys.
Merge pull request #13 from waisbrot/travis-tweak

clean up .travis.yml a little
Merge pull request #12 from waisbrot/fix-add_note_to_loginscreen

fix the add_note_to_loginscreen recipe