Added Travis CI dotfile
Added Travis CI dotfile

Created a new `.travis.yml` with required `language`, `xcode_project`, & `xcode_scheme` settings.  Also added `osx_image` setting with current newest Xcode supported by Travis, `xcode9.2`.
capnslipp/NilCoalescingAssignmentOperators,capnslipp/NilCoalescingAssignmentOperators