Travis: don't test with both gcc and clang

In practice, we hardly ever get different outcomes, so there is no
gain in running tests with different compilers.

Experimentally, with the builds and tests we currently do and with the
compiler versions on a Travis Ubuntu 16.04, gcc jobs are significantly
faster than clang jobs (13 min vs 24 min). So use gcc.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.travis.yml

@@ -1,7 +1,5 @@
 language: c
-compiler:
-- clang
-- gcc
+compiler: gcc
 sudo: false
 cache: ccache