Support for Swift 4!

Swift 4 support (#8)

* - convert to swift 4

* - fixed warnings

* - update tvOS to swift 4
- update OS X to swift 4
- update watch OS to swift 4
- update travis yml config file
- update swift-version

* - remove mplicitly unwrapped optional

* - update variables in travis yml

* - add ruby-version file

* - install cocoapods on travis

* - fix apple tv device for travis ci