Needed to configure differently on Rails 6

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I’m not sure whether it’s that I’m on Rails 6 (and Ruby 2.6.5 on macOS 10.14.6) but when I followed the installation instructions, I was unable to run rspec because it said JsonapiExpectations wasn’t found. I fixed it by using require: false in the Gemfile and adding require "jsonapi_expectations" to the top of spec_helper.rb.

I don’t know if this is something that would be true for everyone so I didn’t make this a documentation PR, but if anyone is trying this out, maybe it’ll help them.

Thanks for your work on this!