Removed hard-coded commit number.

With the previous source definition (`s.source   = { :git => 'https://github.com/danielamitay/DAKeyboardControl.git', :commit => '31b3687a3aa1c36b3754e54ea7687d36b9e89084' }
`), everyone that installed DAKeyboardControl via CocoaPods was stuck in a 5 months-old version. Several tickets (including crashes) have been reported and fixed since.