Upgrade Fabric to 1.8.3

So that we can use parallel execution with `env.gateway` again. It was
hanging previously, but now:

    (fabric)➜  fabric-scripts git:(upgrade_fabric_183) ✗ fab -P staging class:elasticsearch do:date
    [elasticsearch-1.backend] Executing task 'do'
    [elasticsearch-2.backend] Executing task 'do'
    [elasticsearch-3.backend] Executing task 'do'
    [elasticsearch-3.backend] run: date
    [elasticsearch-2.backend] run: date
    [elasticsearch-1.backend] run: date
    [elasticsearch-1.backend] out: Wed May 14 12:54:22 UTC 2014
    [elasticsearch-1.backend] out:

    [elasticsearch-3.backend] out: Wed May 14 12:54:22 UTC 2014
    [elasticsearch-3.backend] out:

    [elasticsearch-2.backend] out: Wed May 14 12:54:22 UTC 2014
    [elasticsearch-2.backend] out:

I can't see any backwards incompatible changes in the intermediate releases:

    https://fabric.readthedocs.org/en/1.8/changelog.html

This will come in useful for the Elasticsearch upgrade where we want all three
machines to run Puppet and come back ASAP.
Merge pull request #45 from alphagov/upgrade_fabric_183

Upgrade Fabric to 1.8.3