Use context passed in as parameter instead of +MR_contextForCurrentThread

Previously, this method was using the context supplied as a parameter
to create the fetch request, but then used `[NSManagedObjectContext
MR_contextForCurrentThread]` to create the
`NSFetchedResultsController`.
It will now correctly use the provided context for both the fetch
request and the `NSFetchedResultsController`.
Merge pull request #871 from Alydyn/develop

Use passed context parameter