Better index version downloads for the update script

The query used to grab the next batch of unprocessed version downloads
is now taking on average 9 seconds to run. It can take as much as 30
seconds. I suspect this is because the size of the `processed` index, or
some other measurement PG is using changed so it decided that index
wasn't the right one to use.

That index wasn't very helpful anyway (it was just the `processed`
column). We can do better by indexing the id with the where clause.

`EXPLAIN ANALYZE` results:

Before:

```
                                                                             QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.09..16204.73 rows=1000 width=21) (actual time=39279.902..39298.408 rows=1000 loops=1)
   ->  Index Scan using version_downloads_pkey on version_downloads  (cost=0.09..199171.32 rows=12291 width=21) (actual time=39279.900..39298.335 rows=1000 loops=1)
         Index Cond: (id > 0)
         Filter: (NOT processed)
         Rows Removed by Filter: 8544073
 Planning time: 0.369 ms
 Execution time: 39298.459 ms
(7 rows)
```

After:

```
                                                                                QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.06..95.89 rows=1000 width=21) (actual time=0.016..0.796 rows=1000 loops=1)
   ->  Index Scan using index_version_downloads_not_processed on version_downloads  (cost=0.06..1175.30 rows=12264 width=21) (actual time=0.016..0.732 rows=1000 loops=1)
         Index Cond: (id > 0)
 Planning time: 0.123 ms
 Execution time: 0.854 ms
```

Author: sgrif

migrations/2018-04-13-170549_properly_index_version_downloads/down.sql

@@ -0,0 +1,4 @@
+CREATE INDEX index_version_downloads_processed
+  ON version_downloads (processed)
+  WHERE processed = FALSE;
+DROP INDEX index_version_downloads_not_processed;

migrations/2018-04-13-170549_properly_index_version_downloads/up.sql

@@ -0,0 +1,4 @@
+CREATE UNIQUE INDEX index_version_downloads_not_processed
+  ON version_downloads (id)
+  WHERE processed = FALSE;
+DROP INDEX index_version_downloads_processed;