In travis, remove c++ version check.

My version checks were returning misleading information. I wanted to know that
g++-4.8 and clang were being used (instead of gcc 4.6, for example). In both
checks, the version returned was not what I expected. But CMake is still using
the compilers I wanted it to use. We've just enforced this through a different
means than I use on my own machine: setting the $CXX environment variable.