Change redis installation strategy

Redis appears to maintain packages here, dating back to 2010, though it doesn't seem it gets new versions very frequently -- I suspect about as frequently as the redis apt repository gets new versions.

To upgrade redis from the old deb (2.4.something in production right now I think), you'll want to:

1. Shutdown the slave
2. `redis-cli shutdown save`  <== will cause redis to persist its state and shut down
3. `sudo apt-get remove redis-server redis-tools libjemalloc1`
4. `puppet agent -t`
5. Startup redis
6. **Delete the dump.rdb file that was saved as a result of the `save`**
7. Restart HAProxy (to let the app start using the master again)
8. Perform steps 3-6 on the slave
9. Restart HAProxy (to let it acknowledge the save is up)

The `redis-cli shutdown save` technique can probably be used moving forward rather than the sync-to-slave-sync-back-to-master approach we've been taking in the past. Only downside is that restart takes longer with a large memory footprint, but realistically shouldn't be an issue at the moment.