Moved the WPMU DEV Membership code under the WPMUDEV namespace.
Added an check around the 'M_register_gateway' function call, is no longer part of Membership 3.5.

Fatal error: Call to undefined function M_register_gateway()
Tweak - WPMU DEV Membership - Fixed error after update to Membership 5.3+, we don't support 3.5+ yet.
Merge branch 'release/2.6.7'