Only disable WP_DEBUG on PHP 5.5 Travis CI tests when running WP 3.7

This disabling is meant to allow builds to pass even in the presence of mysqli-
related debug notices. But later versions of WP either suppress the notice, or
support mysqli natively.

See #5708

Props netweb

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8515 cdf35c40-ae34-48e0-9cc9-0c9da1808c22