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
Revert "Remove gemfury source from Gemfile."

This reverts commit fc301b38f7faf75be8a6101343fa4d83ac278563.
Merge pull request #98 from alphagov/fix-broken-tests

Revert frontend toolkit upgrade, back to 0.20.0 (from 1.6.0)