Update bigbluebutton_rails

It adds a method 'bigbluebutton_can_create?' that allows us to customize
the users that can or cannot start a meeting. Previously only moderators
were allowed (and that's how the method is behaving for now).
Fix failing specs
Merge branch 'master' into recordings
Rooms' 'meetingid' are now fixed and globally unique

Update made in bigbluebutton_rails, see #734 and #735.
Merge remote-tracking branch 'upstream/recordings' into recordings