Add a migration that adds a missing uniqueness constraint that was defined in the model

The uniqueness constraint was added in 599c881714, but it
was not part of the migration.

module_build_service/migrations/versions/65ad4fcdbce6_add_missing_uniqueness_constraint.py

@@ -0,0 +1,24 @@
+"""Add a missing uniqueness constraint to module_builds_to_module_buildrequires
+
+Revision ID: 65ad4fcdbce6
+Revises: 6d503efcd2b8
+Create Date: 2019-04-30 19:56:38.447195
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '65ad4fcdbce6'
+down_revision = '6d503efcd2b8'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    with op.batch_alter_table('module_builds_to_module_buildrequires') as b:
+        b.create_unique_constraint('unique_buildrequires', ['module_id', 'module_buildrequire_id'])
+
+
+def downgrade():
+    with op.batch_alter_table('module_builds_to_module_buildrequires') as b:
+        b.drop_constraint('unique_buildrequires', type_='unique')