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

The xenial Travis-CI environment uses a g++ version that is old enough
that it doesn't use C++11 or newer by default, so we have to force it by
setting CXX to "g++ -std=c++11".

.travis.yml

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