[travis] fast_finish + allow_failures for OSX builds
[travis] fast_finish + allow_failures for OSX builds

Travis CI has been experiencing maintainance problems with the OSX infrastructure
and it has become too slow. It's better we temporarily enable 'allow_failures'
in combination with 'fast_finish' for the OSX builds, so that the CI goes green
as soon as the other builds have completed.

fonttools/fonttools,googlefonts/fonttools