Use MongooseIM commit w/fixed deps

This ought to fix build issues on Travis caused by Folsom not starting
up properly.
Add missing escalus_user_db behaviour
Use escalus_connection:client() instead of ...:transport()

`escalus_connection:transport()` doesn't exist anymore.
Reexport type escalus_config:config/0 as escalus:config/0

Since 1658f58 this type is used but not defined - Dialyzer weeps.
Add TODO about escalus_ejabberd as escalus_user_db