.travis.yml: exclude gcc from MacOS X CI builds.

Contemporary Xcode gcc is a front-end to clang, so that explicit
gcc build is actually redundant on MacOS X.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

.travis.yml

@@ -51,6 +51,8 @@ matrix:
     exclude:
         - os: osx
           compiler: clang-3.6
+        - os: osx
+          compiler: gcc
         - os: osx
           compiler: gcc-5
         - os: osx