Merge th3flyboy-bulletupdate.
Moved rolling friction attribute last in EC_RigidBody to retain protocol wire compatibility (new attributes should be added last)
Removed deprecated Bullet build config variables.
Update Mac deps build script to fetch Bullet 2.81.
Disable postfixes from Bullet library names.
enabled is now an attribute in EC_Sky.
Define rollingFriction last in EC_RigidBody header, so that attribute indices stay unchanged.
Added note of how attributes should be added to maintain protocol compatibility.
Fixed Bullet build on OS X.