fix #174 Drop mongodb roles index "name_1" in _forwardMigrate
fix #174 Drop mongodb roles index "name_1" in _forwardMigrate

See #174

**WARNING** : throw an exception if index "name_1" does not exists. Any reason to think this might happen? 
In that case, might want to surround with a try catch block and print a warning.
alanning/meteor-roles