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. Clang tends to be
slightly pickier, so keep Clang and drop GCC.

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

Author: gilles-peskine-arm

.travis.yml

@@ -1,7 +1,6 @@
 language: c
 compiler:
 - clang
-- gcc
 sudo: false
 cache: ccache