MR_clearContextForCurrentThread was not clearing Context Cache Version
which would cause an assertion failure on the next call to
MR_contextForCurrentThread

Fixes #728
Merge pull request #729 from darend/develop

Clear context cache version (`kMagicalRecordManagedObjectContextCacheVersionKey`) in `MR_clearContextForCurrentThread`
Merge branch 'develop' of github.com:magicalpanda/MagicalRecord into develop