Fix unresolvable dependencies on CI

CI (or deleting Gemfile.lock) and then running "bundle install", would fail
with this message:

[31mBundler could not find compatible versions for gem "multi_json":
  In Gemfile:
    gemfury (= 0.4.12) ruby depends on
      multi_json (< 1.4, >= 1.0.2) ruby

    govuk_content_models (>= 0) ruby depends on
      multi_json (1.6.1)