Add jasmine-rails to run JavaScript tests

- Add `jasimine-rails` gem to `Gemfile`
- Mount test runner on `/spec` so tests can be run in browser
- Add `jasmine_rails.rake` rake task so that tests are run by default
  when you call `bundle exec rake`
- Add copy of jQuery to helpers. This is normally included on pages
  using static/slimmer but won't be available in tests
- Add config for `jasmine-rails`

This commit+setup courtesy of https://github.com/alphagov/finder-frontend/commit/c3bd8b2098c20bff525b53728a9c8d67f13e4aae