Cleanup and linting
Release 1.5.1
Produce erroring migration test
explicitly override db_constraint in `CustomForeignKey.__init__()`
don't bother generating reverse relation for `CustomForeignKey`
Added 'makemigration' command to tests
Make db_constraint unflagging backward compatible
Cleanup, fix test declaration
Merge pull request #128 from treyhunner/fix-additional-migration

More fixes for `CustomForeignKey`
Release 1.5.2