Add back Ruby 2.0 + 2.1 and update patch levels
Add back Ruby 2.0 + 2.1 and update patch levels

Adds back Ruby 2.0 and 2.1 to the build matrix, supported by making sure
we use a Bundler < 2.0 as suggested by Travis (it turns out that the 2.0
release broke *a lot* of projects):

https://docs.travis-ci.com/user/languages/ruby/#bundler-20

Also update patch levels to the latest for the Ruby versions that
support them [1]

[1] https://www.ruby-lang.org/en/downloads/releases/

brandur/json_schema