Revert "Try to update to GCC 6 on Travis."

This reverts commit 55e1dd6203.

.travis.yml

@@ -11,13 +11,13 @@ addons:
     sources:
     - ubuntu-toolchain-r-test
     packages:
-    - gcc-6
-    - g++-6
+    - gcc-4.8
+    - g++-4.8
 
 install:
   - build/InstallMono.sh
   - build/InstallNugets.sh  
-  - if [ "$TRAVIS_OS_NAME" = "linux" ] && [ "$CXX" = "g++" ]; then export CXX="g++-6" CC="gcc-6"; fi
+  - if [ "$TRAVIS_OS_NAME" = "linux" ] && [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then PREMAKE=build/premake5-linux-64; else PREMAKE=build/premake5-osx; fi
   - $PREMAKE --file=build/scripts/LLVM.lua download_llvm