Fix CI Failure JSON gem with 'ruby-head'

[ksylvest]

Ruby JSON appears to have changed parsing of errors between v2.6 and v2.7. For example - when running:

require 'json'

data = {}
data['cycle'] = data
JSON.dump(data)

With Ruby v2.6.5

This raises a SystemStackError.

With Ruby v2.7.0

This causes fatal (machine stack overflow in critical region)

This is causing Travis CI to fail:

https://travis-ci.org/getsentry/raven-ruby/jobs/636721290?utm_medium=notification&utm_source=github_status

https://github.com/getsentry/raven-ruby/blob/master/spec/raven/json_spec.rb#L75

[ksylvest]

Note: confirmed this isn't a ruby version issue - but actually a JSON gem version issue:

With json v2.1.0

This raises a SystemStackError.

With json v2.3.0

This causes fatal (machine stack overflow in critical region)

• Ruby 2.7.0 has 2.3.0 as default
• Ruby 2.6.5 has 2.1.0 as default

[ksylvest]

Closed: https://rubygems.org/gems/sentry-raven/versions/3.0.0 :)