Bump capybara to 2.0.2 and poltergeist to 1.1.0

This gives us access to some methods that can help with testing things
that we're working on in the (near) future.

Most of these changes involve one of two things:

  - The way that 'within' works in capybara 2.0.2 has changed to take
    a set of elements instead of the first match. The 'first' method
    has been used in these cases.
  - The way that querying the title element works has also changed.
    Instead it needs to be mapped to Element.native.text. This change
    has shown an error in the way that our tests worked previously in
    that they appear to have been lying to us(!)
Merge pull request #275 from alphagov/bump-capybara-and-poltergeist

Bump capybara to 2.0.2 and poltergeist to 1.1.0