Fix travis-ci
Fix travis-ci

The latest version of bundler (version 2) doesn't work with Ruby
versions before 2.3. Travis fails to install the appropriate version of
bundler for older versions of Ruby, but seemingly only on macOS. Travis
support suggested adding the following to `.travis.yml`.

``` yaml
before_install:
  - gem uninstall -v '>= 2' -i $(rvm gemdir)@global -ax bundler || true
  - gem install bundler -v '< 2'
```

This change performs that action, but only if `RUBY_VERSION` is less
than 2.3.

rubys/nokogumbo,rubys/nokogumbo,rubys/nokogumbo,rubys/nokogumbo