travis-cl: disable gcc-6 build

.travis.yml

@@ -87,20 +87,20 @@ matrix:
               - p7zip-full
           compiler: gcc-5
           #gcc-5 (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204
-        - os: linux
-          addons:
-            apt:
-              sources:
-              - ubuntu-toolchain-r-test
-              packages:
-              - gcc-6
-              - libsdl2-mixer-dev
-              - libpng-dev
-              - libgl1-mesa-dev
-              - libgme-dev
-              - p7zip-full
-          compiler: gcc-6
-          #gcc-6 (Ubuntu 6.1.1-3ubuntu11~14.04.1) 6.1.1 20160511
+#        - os: linux
+#          addons:
+#            apt:
+#              sources:
+#              - ubuntu-toolchain-r-test
+#              packages:
+#              - gcc-6
+#              - libsdl2-mixer-dev
+#              - libpng-dev
+#              - libgl1-mesa-dev
+#              - libgme-dev
+#              - p7zip-full
+#          compiler: gcc-6
+#          #gcc-6 (Ubuntu 6.1.1-3ubuntu11~14.04.1) 6.1.1 20160511
         - os: linux
           compiler: clang
           #clang version 3.5.0 (tags/RELEASE_350/final)