Travis CI: enable C++ unit tests

The tests should be able to run in Docker, and now when they are no longer
failing, we can enable them.

.travis.yml

@@ -34,7 +34,7 @@ before_install:
     - export PLAY_TEST=true
     - export MP_TEST=true
     - export WML_TEST_TIME=15
-    - export BOOST_TEST=false
+    - export BOOST_TEST=true
 
     - if [ "$OPT" = "-O0" ]; then
           export EXTRA_FLAGS_RELEASE="-O0";