Fix collision resolution

Previously, this would just exit if a collision was found since it would
recheck the same short url over and over again.

Additionally, it would not increment the cache counter on successful
short generation since the error would cause the rest of the stanza
to no longer execute
4 jobs for nanderson-fixcollision in 1 minute and 17 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2454
Go-py2.7

00:01:12

passed #2455
Go-py3.4

00:01:16

passed #2456
Go-py3.5

00:01:17

passed #2457
Go-py3.6

00:01:15

72.0%