Only add team_captain_id foreign key if the db backend isn't SQLite (#1048)

* Only add `teams.team_captain_id` foreign key if the db backend isn't SQLite. SQLite does not support ALTER for the manipulation of columns/constraints. 
* Closes #1041

migrations/versions/b5551cd26764_add_captain_column_to_teams.py

@@ -30,7 +30,11 @@ users_table = table('users',
 def upgrade():
     # ### commands auto generated by Alembic - please adjust! ###
     op.add_column('teams', sa.Column('captain_id', sa.Integer(), nullable=True))
-    op.create_foreign_key('team_captain_id', 'teams', 'users', ['captain_id'], ['id'])
+
+    bind = op.get_bind()
+    url = str(bind.engine.url)
+    if url.startswith('sqlite') is False:
+        op.create_foreign_key('team_captain_id', 'teams', 'users', ['captain_id'], ['id'])
 
     connection = op.get_bind()
     for team in connection.execute(teams_table.select()):