Travis: reactivate warnings about different API and schema versions

Since the Operations meeting on 2019-08-27 the bumping of Ensembl API
and DB-schema version numbers has been an Ensembl-wide policy.
Therefore, have Travis take notice of possible discrepancies in this
respect.

Note that simply removing the before_script line creating the file
disabling these checks is in general NOT sufficient - it might have
been left behind by a previous build in the event of runner re-use. It
is safer to make sure it does not exist regardless of initial conditions
of the runner.