TravisCI: remove gcc-5 test, gcc-5 package is gone

.travis.yml

@@ -71,20 +71,6 @@ matrix:
               - gcc-4.9
           compiler: gcc-4.9
           #gcc-4.9 (Ubuntu 4.9.3-8ubuntu2~14.04) 4.9.3
-        - os: linux
-          addons:
-            apt:
-              sources:
-              - ubuntu-toolchain-r-test
-              packages:
-              - libsdl2-mixer-dev
-              - libpng-dev
-              - libgl1-mesa-dev
-              - libgme-dev
-              - p7zip-full
-              - gcc-5
-          compiler: gcc-5
-          #gcc-5 (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204
         - os: linux
           addons:
             apt: