Enhance rendering specs + update rspec and prawn

I'm going to make some modifications to allow users to customise the default invoice styling more easily (on my fork - will create pull request if there is interest).

Before embarking on this refactoring I made a couple of changes which I thought might benefit the main repo:

1. Fixed a broken spec expectation to get the test suite working.

2. Improved the rendering specs so that they now compare rendered output with an actual expected PDF asset, rather than simply testing whether there _is_ any output, as is currently the case. This way I'll be sure that my changes don't unintentionally affect how invoices are currently rendered.

3. Upgraded the rspec and prawn gems to latest stable versions. The prawn upgrade did change the PDF file a little, but aside from one issue with bold cells which I fixed, nothing that caused any visible changes to the rendered invoices.