Tidy up Gemfile

This removes some unused/unnecessary gems and also removes staging
from the development/test group.

brakeman - is an independent gem that doesn't need to be loaded in-app
boomerang_rails - no longer used
crack - webmock now uses a safe version
minitest - Ruby 1.9.3 ships with version 2.5.1 already
Update cucumber and cucumber-rails gems to latest
More gem updates

Kaminary has renamed `num_pages` to the more sensible `total_pages`
Remove faker gem
WIP Update capybara/capybara-webkit

Mainly involved disambiguating a few selectors as capybara 2 is more
strick and will raise an error if a selector matches more than one
element on the page, which is pretty sensible when you think about it.

There are still a handful of failing features to sort out, including
the javascript-based ones.