Fix typo setting mysql engine

For the runs table migration the mysql_engine param was set as true
which obviously isn't a valid mysql_engine. This commit fixes it
and sets it to the proper innodb.

subunit2sql/migrations/versions/1f92cfe8a6d3_create_runs_table.py

@@ -37,7 +37,7 @@ def upgrade():
                     sa.Column('passes', sa.Integer()),
                     sa.Column('run_time', sa.Float()),
                     sa.Column('artifacts', sa.Text()),
-                    mysql_engine=True)
+                    mysql_engine='InnoDB')
 
 
 def downgrade():