travis: Use concurrency = 2 for make

* "Travis CI VMs run on 1.5 virtual cores" [1]

[1] http://docs.travis-ci.com/user/speeding-up-the-build/
travis: Log gcc version

* Ask GCC to print it's version.  Travis-CI would do this automatically
  if there was a way to install gcc before `$CC --version` is
  automatically run.