Add hmrc crests and colour

• add crest option to admin
• add crest pngs
• add colour variable to tool kit (pull request here https://github.com/alphagov/govuk_frontend_toolkit/pull/49)
• add hmrc mixin to colours css
• add relevant css to orgs css

https://www.pivotaltracker.com/story/show/53235311
Update Gemfile to use new govuk_frontend_toolkit

This contains the required HMRC colour.
Merge pull request #662 from alphagov/add-hmrc-crests-and-colour

Add hmrc crests and colour

https://www.pivotaltracker.com/story/show/53235311