travis-ci: set DEVELOPER knob for Linux32 build

The Linux32 build was not build with our strict compiler settings (e.g.
warnings as errors). Fix this by passing the DEVELOPER environment
variable to the docker container.

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

.travis.yml

@@ -61,6 +61,7 @@ matrix:
         - >
           docker run
           --interactive
+          --env DEVELOPER
           --env DEFAULT_TEST_TARGET
           --env GIT_PROVE_OPTS
           --env GIT_TEST_OPTS