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>

.travis.yml

@@ -39,6 +39,11 @@ env:
 
 matrix:
   include:
+    - env: GETTEXT_POISON=YesPlease
+      os: linux
+      compiler:
+      addons:
+      before_install:
     - env: Windows
       os: linux
       compiler: