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`.
Added a 3rd condition for the migration error case
Add workaround for Travis builds failing — see https://github.com/travis-ci/travis-ci/issues/2836

Conflicts:
	.travis.yml
Update podspec to 2.3.0-beta 5
Update logging documentation for 2.3.0
Merge pull request #871 from Alydyn/develop

Use passed context parameter
Merge pull request #881 from birthdayalarm/develop

Check for NSMigrationError alongside NSPersistentStoreIncompatibleVersionHashError and NSMigrationMissingSourceModelError
Merge branch 'developMain' into NSOperationQueuesMigration
no message
magical record migration to NSOperationQueue,

    - add tests for background stack creation,
    - add defence code
    - add documentation,

now to init core data on background threads can used:

            [MagicalRecord MR_setMainThread:backgroundQueue];
            [MagicalRecord setupAutoMigratingCoreDataStack];