Travis and cpp11 updates

1. Cleaned up travis file; now building 8 times: c++11 on/off, build/release, and gcc/clang.
2. clang c++11 now passes on travis, b/c I'm using an up-to-date libstdc++.
3. On OSX, if using clang and c++11, we use the libc++ standard library instead. Tested on a mac and all tests pass.

Travis takes 19-20 minutes to run all tests now. Perhaps running tests for debug mode is overkill? I decided to do this because OpenSim's tests were failing in debug even though the tests passed in release.