Fix multiple db_migrations heads (#6560)

superset/migrations/versions/a7ca4a272e0a_.py

+"""empty message
+
+Revision ID: a7ca4a272e0a
+Revises: ('3e1b21cd94a4', 'cefabc8f7d38')
+Create Date: 2018-12-20 21:53:05.719149
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'a7ca4a272e0a'
+down_revision = ('3e1b21cd94a4', 'cefabc8f7d38')
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    pass
+
+
+def downgrade():
+    pass