Support swift 4.2 (#4)

* Update code to swift 4.2

* Update test project to swift 4.2

* Update Travis config

* Fix Travis pod update

* Temporary allow warnings to pass `Quick` pod warning
`WARN  | xcodebuild:  Quick/Sources/Quick/Behavior.swift:7:21: warning: static declarations are implicitly 'final'; use 'public' instead of 'open'`

Fix swift version requirement (#3)

* Modernize `Podfile`

* Fix swift version requirement

* Fix hardcoded podspec source tag