Upgrade RethinkDB to v2.2.0

Revert "Temporarily revert rethinkdb upgrade"

Turns out that despite the fact that we were running rethinkdb version 1.16,
we still had indexes which were created in 1.13.0. There is not a direct
migration path from 1.13.0 indexes straight to 2.2.0. Instead I had to migrate
first to 2.0.5 (Note 2.0.0 SHOULD have worked but there was a bug
https://github.com/rethinkdb/rethinkdb/issues/4775 which we seem to have been
subject to). Once I was on 2.0.5 I was able to rebuild the indexes. From there
I was able to bring us back to 2.2.0 and things worked again.

For the curious, here's the error we were seeing:

    2016-04-15T05:42:01.393253255 0.010778s notice: Running rethinkdb 2.2.0~0trusty (GCC 4.8.2)...
    2016-04-15T05:42:01.407714269 0.025237s notice: Running on Linux 4.4.0-x86_64-linode63 x86_64
    2016-04-15T05:42:01.407772464 0.025290s notice: Loading data from directory /var/lib/rethinkdb/instance1/data
    2016-04-15T05:42:01.419155695 0.036673s notice: Migrating cluster metadata to v2.2
    2016-04-15T05:42:01.875603265 0.493121s error: Found a secondary index from unsupported RethinkDB version 1.13.  You can migrate this secondary index using RethinkDB 2.0.