touchParent wont work if table a name is specified in the model. How to make it dynamic for all relationships?

[deeputvr]

Steps to recreate:

1. create a user model. Give $table = 'test_users';
2. create belongsToMany relation to roles model
3. Now use ->sync(...) to assign some roles
4. See if touch parent worked. Since the checking is based on old code, the feature is not working
5. We needed to call setTouchedRelations() to work this.
   We will have many tables and relations in the project. It wold be great if it handles automatically.

framework/src/Illuminate/Database/Eloquent/Relations/BelongsToMany.php

Line 1142 in ccefc76

protected function guessInverseRelation()