Run one Travis configuration with HASH_CODE enabled

The existing regression tests would have uncovered the problem if only
they had been used in this configuration.

Author: tautschnig

.travis.yml

@@ -105,7 +105,7 @@ jobs:
         - export PATH=$PATH:/usr/local/opt/ccache/libexec
       env:
         - COMPILER="ccache clang++"
-        - EXTRA_CXXFLAGS="-Qunused-arguments -fcolor-diagnostics"
+        - EXTRA_CXXFLAGS="-Qunused-arguments -fcolor-diagnostics -DHASH_CODE"
         - CCACHE_CPP2=yes
 
     # Ubuntu Linux with glibc using g++-5, debug mode