Travis-CI support OSX and GCC-4.8

[Travis-CI support GCC-4.8 and XCode 6.1](http://blog.travis-ci.com/2014-11-03-xcode-61-beta/), travis-ci/travis-ci#2423
- Permitted build failures for `osx` and `linux: clang`.
- Excluded from the matrix construction `osx: clang` and `linux: clang`.