Upgrade to Kibana3 c/o kibana-rack

Use a project called kibana-rack to provide us Kibana3 in the same way that
we ran Kibana2, protected by signon.

- https://github.com/tabolario/kibana-rack

This appears to Just Work. The one significant difference is that this app
is now also responsible for reverse proxying access Elasticsearch for a
handful of methods and endpoints:

- https://github.com/tabolario/kibana-rack/blob/v0.1.1/lib/kibana/rack/web.rb#L46-58

I don't think that's a big risk though, since it's:

- limited to people with production access
- a reasonbly limited number of methods/endpoints
- a separate ES cluster
Set ruby-version to 2.1.2

Per the requirements listed here:

- https://github.com/tabolario/kibana-rack#requirements

This is available to us on GOV.UK since:

- gds/puppet@bf2669a25e5fd24695100af81b385ad61d1d2f6b
Use ES_HOST instead of ELASTICSEARCH_SERVERS

Kibana only currently supports a single Elasticsearch endpoint. It has no
knowledge about clusters. So change the environment variable to reflect
this. gds/alphagov-deployment:kibana will need updating too.

NB: We could wrap this with govuk_elasticsearch::local_proxy in gds/puppet
if it becomes a problem.
Redirect / to /kibana

kibana-rack only serves content from /kibana. Requests for / return a 404
from Sinatra. So handle the redirect in our AuthWrapper.
Vendor logstash.json dashboard as default

We're going to need to modify this to change the index pattern. So vendor it
and set it to the default (so save people needing to click through the
"welcome" page).
Change index pattern to logs-

To match our indexes. Which aren't called logstash-
Set dashboard to use UTC time

Per our Kibana2 config in c03ed9a.
Show @timestamp and @message fields by default

Per our Kibana2 config in c89a1db.
Merge pull request #4 from alphagov/76656336-kibana3_with_kibana_rack

[#76656336] Upgrade to Kibana3 c/o kibana-rack