i#1962 Travis CI: use Trusty
i#1962 Travis CI: use Trusty

Switches to the Trusty environment for a more recent CMake.

Review-URL: https://codereview.appspot.com/299540043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio