Added travis os x support

Inspired from https://github.com/flowgrind/flowgrind/blob/master/.travis.yml
[travis] Adding installation of CMake on OS X 

To address issue https://github.com/travis-ci/travis-ci/issues/2408 .
[travis] allow failure on osx
[travis] use make generator both on linux and os x, and use Xcode only on os x
[travis] fix syntax error
[travis] avoid using .. in travis
[travis] added missing then
Merge pull request #55 from traversaro/patch-2

Adding Travis OS X support