Set up Unit Testing w/ Travis CI

Adds
- .travis.yml for Travis environment configuration
- travis.rb build execution script

Unit tests are run via xcodebuild and formatted by xcpretty.
Updated CSE key for development mode
Make Travis only build PRs
Don't show AVCaptureDeviceInput warning on simulators
Version bump: 1.5.6

[Enhancements]
* Correct CSE key for development mode