TravisCI: also remove gcc-6

.travis.yml

@@ -57,21 +57,6 @@ matrix:
               - gcc-4.8
           compiler: gcc-4.8
           #gcc-4.8 (Ubuntu 4.8.5-2ubuntu1~14.04.1) 4.8.5
-        - os: linux
-          addons:
-            apt:
-              sources:
-              - ubuntu-toolchain-r-test
-              packages:
-              - libsdl2-mixer-dev
-              - libpng-dev
-              - libgl1-mesa-dev
-              - libgme-dev
-              - p7zip-full
-              - gcc-6
-          compiler: gcc-6
-          env: WFLAGS="-Wno-tautological-compare"
-          #gcc-6 (Ubuntu 6.1.1-3ubuntu11~14.04.1) 6.1.1 20160511
         - os: linux
           addons:
             apt: