Revert ".travis.yml: Force C++ standard language version"

This reverts commit 0585e053.

Now that we autodetect any needed flags to enable C++11 support with the
AX_CXX_COMPILE_STDCXX macro, we no longer need to force this in
.travis.yml.

.travis.yml

@@ -13,7 +13,6 @@ addons:
 
 env:
   global:
-    - CXX="g++ -std=c++11"
     - PROTOBUF_VERSION=3.6.1
     - PKG_CONFIG_PATH=$HOME/protobuf-$PROTOBUF_VERSION-bin/lib/pkgconfig