added -O0 to gcov builds

Added the -O0 optimization flag to builds with ENABLE_GCOV set to ON.
This should help equalize the coverage analysis output for gcc and llvm.
coveralls now skips CMakeFiles

The coveralls step in .travis.yml included the CMakeFiles directory, which
tends to be created by CMake when using LLVM as compiler. This directory is
now being skipped.