Updated the ACF v4 class to use the updated code found in the ACF v5 Class
Fixed a silly mistake related to allowing Null for a Nav Menu Field. Basically, it was storing the string "null" when you don't select a menu, that's taken care of now.
I forgot to add a default value for the Menu Container field, so I added 'div' as the default value. If you upgraded from 1.0.0 to 1.1.0 and had WP_DEBUG enabled, you would receive a warning about an unknown index. Since I like debug mode to run without warnings, I fixed this.