Fix build.

The build started failing on Travis CI [1]. The problem was that neither
the jQuery nor the jQuery UI libraries were being loaded into the page.
I'm not sure why this has only just become a problem.

In normal operation of the Chrome extension these libraries are loaded via
the extension's manifest file, but in the integration specs we're taking a
bit of a shortcut, so we need to load them manually.

[1]: https://travis-ci.org/freerange/freeagent_transaction_explainer/builds/39270289
Remove tags which load external JS and CSS from fixture HTML.

While it would be nice to keep the page as close to the original as possible
it doesn't seem good to have the specs rely on these external resources.
DRY up integration specs.