Bump minimum WordPress requirement to 3.6

BuddyPress 2.0 introduces a number of features that require WP 3.6, including
some use of wp_unslash() and integration with the Heartbeat API.

See #5328, #5431 et al

Props DJPaul

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7991 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Remove WP 3.5.x from travis-ci testing matrix

BP 2.0 will require WP 3.6+

Also bumps WP 3.8.x tests to run against 3.8.1 rather than 3.8.0.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7992 cdf35c40-ae34-48e0-9cc9-0c9da1808c22