Update .travis.yml
Use the build matrix instead of manual abort

 - To avoid GCC + satinizers build, use the build matrix instead of a manual abort.
 - The abort on using libbc++ with GCC is not needed, since libc++ is enabled with clang only.
I'm not very good at YAML, sorry
Merge pull request #66 from Manu343726/patch-1

Enable GCC 4.9 travis build