fix Rails 4.2/5 test compatibility

Hi,

I fixed some of the broken Rails tests:

- `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`).
- as rails/master now points to 5.0.0:
  - exclude 1.9/2.0 + rails-master from build matrix (Rails 5 will require Ruby >= 2.1)
  - simplify conditional in Gemfile

Some tests are still broken:

- jRuby closes streams an script tag (see [here](https://travis-ci.org/dmke/slim/jobs/45027520#L174-L191)). I just don't know where to start here...
- Rubinius fails to install (which seems to be an issue with Travis?)