Integrating Mac OS X and clang compiler into Travis CI (#601)

changes: 
- .travis.yml:
  - added matrix for different osx's, since enumeration style only works for `env` and `compiler`

- scripts/travis/build.sh:
  - changed getting compiler version from ``${CXX} -dumpversion`` to ``${CXX} --version | grep -m1 ""`` 
  - added `--oversubscribe` flag to `mpiexec` on Mac to allow more processes on a node than processing elements
  - added `--mca btl tcp,self` flag for Open MPI to use the "tcp" and "self" BTLs for transporting MPI messages on Mac