Upgrade admin gem to 1.1.6

* Upgrade jQuery to 1.11.1
* Upgrade Bootstrap to 3.2
Upgrade Capybara and Poltergeist.

 - Update Capybara and Poltergeist to use same versions as Publisher.
 - Update features to accomodate Capybara changes from v1.* to v2.*
 - Add PoltergeistHelpers module to provide a workaround to Bootstrap fixed navigation styles.
Add stubbing for related items updates.

Add a support method to cater for cases where related items are being
updated.
Merge pull request #74 from alphagov/bump-admin-gem

Bump admin gem