[1.7.x] Bump version numbers for 1.7 beta 1.
Add a test to remove a foreign key field

This test fails on MySQL side:

OperationalError: (1025, "Error on rename of './test_myapp/test_rfk_rider' to './test_myapp/#sql2-3398-2e8' (errno: 152)")

In MySQL you explicitly have to remove the constraint first.
Fix removal of a ForeignKey field