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
Update capybara and capybara-webkit

There are a few significant changes in Capybara 2.1 that have to be
dealt with:

1. It will now complain if your selector is ambiguous and matches more
than one element

2. It will not match elements that are not visible

1 has meant having to disambiguate a few selectors by being more
specific. 2. has meant having to be a bit more careful with
JS-enhanced elements. For example, select boxes that are enhanced with
chosen.jquery.js need some special handling.

A couple of finder options come in handy in both regards. Firstly, the
:visible option allows you to specify how strict the visibility
checking is (set :visible to :all and it will also match invisible
elements. Secondly, :match allows you to specify how strict the
ambiguity checking is (specify :match as :first and your finder will
return the first element)

For some good info on the differences and upgrading, see:

http://techblog.fundinggates.com/blog/2012/08/capybara-2-0-upgrade-guide/
http://www.elabs.se/blog/60-introducing-capybara-2-1