Remove package array from Nginx Puppet module

In an attempt to get the tests running on Travis CI - Precise VMs and Puppet
2.7 - which doesn't support chaining using arrays of resources:

    Could not find resource 'Package[nginx]Package[ssl-cert]' for relationship on 'File[/etc/nginx/sites-enabled/default]' on node testing-worker-linux-11-1-462-linux-20-28943308
Skip two remaining unimplemented tests

Rather than making the entire suite fail. We have stories to complete these,
so it's more useful to get meaningful feedback about the other tests in the
meantime.
Pass in Varnish backend address to mock config

So that we can override this to localhost when running on Travis.
Use third-party repo for Varnish on Precise

Precise only has Varnish 3.0.2 which behaves differently from 3.0.5 in
Trusty. So use a third-party repo to match Trusty.

Normally I'd use puppetlabs-apt to do all of this. But it's too much work to
pull in bundler and librarian-puppet for this project.
Setup Travis CI to test PRs against mocks

Using much of the same setup we use in Vagrant. This will give us greater
confidence when reviewing pull requests.
Merge pull request #65 from alphagov/travis_test_prs

Setup Travis CI to test PRs against mocks