Add branch build script

Webhook + CI setup already done
Merge pull request #99 from alphagov/branch-builds

Add branch build script
Revert "update gemfile to latest toolkit version"

This broke some tests depending on assets in older versions of the toolkit

This reverts commit b35fd48a180ddacfdce30aa0d62c7d8695f23437.
and fd60b740bb2af00aca0d3988f1f336ed92bb2d03