chore(.travis.yml): avoid g++-4.8.1 + DEBUG + TCMALLOC

The combination sometimes triggers the following compiler error:

The log says:
               g++-4.8: internal compiler error: Killed (program cc1plus)

See: https://travis-ci.org/leanprover/lean/jobs/34351197

We can try this one again when travis-ci.org switches to Ubuntu 14.04 where we can use either gcc-4.8.3 or gcc-4.9.0.

See: https://github.com/travis-ci/travis-ci/issues/2046