Moar index speedup.

Add a db index to the deployments table.  The index view results in lots of queries of the deployments table which currently doesn't have any indices.

Prior to the index:
```
mysql> explain SELECT `deployments`.* FROM `deployments` WHERE `deployments`.`application_id` = 7 AND `deployments`.`environment` = 'staging' ORDER BY created_at DESC LIMIT 1\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: deployments
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 16742
        Extra: Using where; Using filesort
1 row in set (0.00 sec)
```

With the index:
```
mysql> explain SELECT `deployments`.* FROM `deployments` WHERE `deployments`.`application_id` = 7 AND `deployments`.`environment` = 'staging' ORDER BY created_at DESC LIMIT 1\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: deployments
         type: ref
possible_keys: index_deployments_on_application_id_etc
          key: index_deployments_on_application_id_etc
      key_len: 263
          ref: const,const
         rows: 21
        Extra: Using where
1 row in set (0.01 sec)
```
Merge pull request #51 from alphagov/index_speedup

Moar index speedup.