Add cask for Firefox UX channel build
user
```ruby
class FirefoxUx < Cask
  url 'https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-ux/firefox-29.0a1.en-US.mac.dmg'
  homepage 'http://people.mozilla.org/~jwein/ux-nightly/'
  version '29.0a1'
  sha1 'be5c293d961d40d2c3aa5f6fcd3d973c68667985'
  link 'FirefoxUX.app'
end
```
assistant