Travis: Remove -Werror

We have some unused parameters in some functions - this shouldn't break
the build.

Change-Id: If0c99892a5af2bc8abbfc436c305e12e3e689760
Reviewed-on: http://review.couchbase.org/44243
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>