Fix Travis for unit test compilation errors. (#1651)

Fix Travis for unit test compilation errors. (#1651)

make units does not fail if we have compile error for a unit test, however, make check does.
-Wsuggest-override is now explicitly disabled for unit tests since CxxTest does not add override keywords to the generated source code and thus get a lot of compiler warnings.

Further, this fixes some issues introduced with #1647 due to make units not failing on Travis and fixes the nightly builds.