fix Rails 4.2/5 test compatibility

- ActionDispatch::Integration::RequestHelpers#get and friends require
  a path to start with "/"
- The order of HTML attributes has changed when using form helpers,
  which caused the test to look ugly. To accommodate this, an
  `assert_xpath` helper is introduced (cf. test/rails/test/helper.rb).
- Simplify conditional in Gemfile (as rails/master now points to 5.0.0).