fix(travis): Match compiler prefix

For some reason, for the tests CC is set to gcc and not gcc-5, meaning
the ${CXX} = g++ condition wasn't met, most likely because CXX was g++-5
already

common/travis/environment.sh

@@ -1,8 +1,8 @@
 #!/bin/bash
 # Update compiler flags
-if [ "${CXX}" = "clang++" ]; then
+if [ "${CXX:0:7}" = "clang++" ]; then
   export CXXFLAGS="${CXXFLAGS} -Qunused-arguments"
-elif [ "${CXX}" = "g++" ]; then
+elif [ "${CXX:0:3}" = "g++" ]; then
   export CXX="g++-5"
   export CC="gcc-5"
 fi