Fix mis-matched database migrations

The cancelled job migration originally went in as version `0051`.

But `0051_set_job_status` went in first in
e3fa8bffd7

This meant that Alembic couldn’t work out what order to run the
migrations in, and blew up.

This commit puts the scheduled job migration after all others.

migrations/versions/0053_cancelled_job_status.py

@@ -0,0 +1,22 @@
+"""empty message
+
+Revision ID: 0053_cancelled_job_status
+Revises: 0052_drop_jobs_status
+Create Date: 2016-09-01 14:34:06.839381
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0053_cancelled_job_status'
+down_revision = '0052_drop_jobs_status'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+def upgrade():
+    op.execute("INSERT INTO job_status VALUES ('cancelled')")
+
+def downgrade():
+    op.execute("UPDATE jobs SET job_status = 'finished' WHERE job_status = 'cancelled'")
+    op.execute("DELETE FROM job_status WHERE name = 'cancelled';")