* Added TouchBar support for MacBook Pro

* [macOS] Optionalize use of touchbar

There is a new CMake flag named BONZOMATIC_TOUCHBAR (default ON) to activate or deactivate TouchBar support.

* Add a runtime check to see if the TouchBar API is available. If it's not, do not attempt to create a TouchBar.

* Updated Travis CI to use Xcode 9.3 for its build