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.
3. Remove installation of liblas-dev, which is some LIDAR package (oops).
4. Clang and C++11 uses libc++ now. Added some CMake logic to ensure that, with
Clang and C++11, libc++ exists.
Add Debug builds to Travis.
Only use libc++ on Mac's.

On Ubuntu, using libc++ causes a test failure. To get travis tests to pass,
we also make sure we install gcc-4.8 even when testing with Clang, to get an
updated libstdc++.
Remove debug builds from travis.
Update .travis.yml
Merge pull request #2 from chrisdembia/travis-and-clang-cpp11-gcc48-envars

On travis, only compile with gcc 4.8 if this is a gcc build.
Merge pull request #166 from chrisdembia/travis-and-cpp11-updates

Travis and cpp11 updates