Use rest-client to fetch templates

This will allow us to set extra headers more easily.

Note: this change also removes the option to skip verifying SSL certs.
This is a good thing.  We're using proper certs everywhere, so we should
be validating them.  This option being set is a holdover from the very
early days.
Pass the GOVUK-Request-Id header to static

So that we can trace all calls that result from a single request.
Merge pull request #56 from alphagov/govuk_request_id

Pass the GOVUK-Request-Id header to static when loading templates