Removes C++-compiler-as-C-compiler Travis build

Author: daniel-j-h

.travis.yml

@@ -14,13 +14,6 @@ matrix:
           packages: ['g++-6', 'cmake', 'python3', 'nodejs', 'npm', 'libsfml-dev']
       env: CCOMPILER='gcc-6' CXXCOMPILER='g++-6'
 
-    - os: linux
-      addons:
-        apt:
-          sources: ['ubuntu-toolchain-r-test']
-          packages: ['g++-6', 'cmake', 'python3', 'nodejs', 'npm', 'libsfml-dev']
-      env: CCOMPILER='g++-6' CXXCOMPILER='g++-6'
-
     - os: linux
       addons:
         apt: