travis: Move fastest test configuration first

travis: Move fastest test configuration first

I use Travis CI to check each patch before sending my patchsets.  Most
failures are simple and make all test configurations fail.  However, since
Travis CI runs test configurations in order and given that the first three
configurations are the slowest ones, it takes about twenty minutes to see
the failure.  Moving the fastest test configuration first would allow to
fail earlier.  It would become about 3x faster to fail in case of simple
mistakes.

* .travis.yml (CC=gcc, STACKTRACE=no): Move to first position.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>