Fixes failing test in gis app

The migrations test in the `contrib.gis` app was failing on MySQL due to an incompatible field definition.  This PR fixes that.

http://ci.djangoproject.com/job/Django/database=mysql_gis,python=python2.7/4198/testReport/junit/django.contrib.gis.tests.gis_migrations.test_commands/MigrateTests/test_migrate_gis/