Test against OS X on Travis

[omasanori]

Travis provides an OS X build environment, so we can test against it too. CI on OS X is especially useful for who have no Apple machine for development, like me. :smiley:

Note that the build environment have two GCCs; one is normal GCC, and the other is LLVM-based one (this one must be familiar to OS X folks, I guess). See https://github.com/travis-ci/travis-ci/issues/2423 for details.