Update travis and fix Clang with C++11. Fixes #163.

1. Use Travis env tag to build with and without C++11.
2. General clean up. Move some commands into before_install.
4. Remove installation of liblas-dev, which is some LIDAR package (oops).
5. Clang and C++11 uses libc++ now. Added some CMake logic to ensure that, with
Clang and C++11, libc++ exists.