Remove gcc-4.9 from the travis builds

According to CMake, it does not support templated variables. Strictly
speaking, we added that to the target so that the project defaults
to C++14, but I am planning to use them inside Catch2 anyway.

.travis.yml

@@ -103,14 +103,6 @@ matrix:
 
 
     # GCC builds
-    - os: linux
-      compiler: gcc
-      addons:
-        apt:
-          sources: *all_sources
-          packages: ['g++-4.9']
-      env: COMPILER='g++-4.9' CPP14=1
-
     - os: linux
       compiler: gcc
       addons: