travis-ci: setup "prove cache" in "script" step

travis-ci: setup "prove cache" in "script" step

The command that made the "prove cache" persistent across builds was
executed in the "before_install" step. Consequently, every job that
wanted to make use of the cache had to run this step.

The "prove cache" is only used in the "script" step for the
"make test" command. Therefore, we should configure the "prove cache"
in this step.

This change is useful for a subsequent patch that adds a job which does
not need the "before_install" step but wants to run the "script" step to
execute the tests.

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