language: ruby
rvm:
  - 2.3.1
  - 2.2
  - 2.1
  - 2.0

language: ruby
rvm:
  - 2.4.0
  - 2.3.1
  - 2.2
  - 2.1

Test on 2.4, and drop 2.0
Test on 2.4, and drop 2.0

Recent versions of nokogiri no longer work on ruby 2.0, so drop travis
testing for that (and add the newly released 2.4).

We don't explicitly force a version > 2.0 yet, because it should still
work with an older nokogiri
