travis-ci: add job to run tests with GETTEXT_POISON

travis-ci: add job to run tests with GETTEXT_POISON

Add a job to run Git tests with GETTEXT_POISON. In this job we don't run
the git-p4, git-svn, and HTTPD tests to save resources/time (those tests
are already executed in other jobs). Since we don't run these tests, we
can also skip the "before_install" step (which would install the
necessary dependencies) with an empty override.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>