Drop Failing Spec for Circular References in Hashes

[ksylvest]

Fix for #947.

The JSON 2.3.0 gem changed how parsing errors are raised. For example:

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

With json v2.2.0:
This raises a SystemStackError.

With json v2.3.0:
This causes fatal (machine stack overflow in critical region)

[ksylvest]

@HazAT wanted to followup from the change introduced here: #946

I see a few different options:

1. pin the version of json to 2.2.0 (probably not a great idea since it is a system library - also leads to lots of warnings with 2.7.0)
2. drop the failing test here (https://github.com/getsentry/raven-ruby/blob/master/spec/raven/json_spec.rb#L64-L77)
3. change the test to handle ruby version changes
4. try and get this fixed in https://github.com/ruby/ruby / https://github.com/flori/json (issue: Changes in JSON.dump behaviour with cyclical structures flori/json#404)

Do you have any thoughts? #2 seems like the best option to me (since this test is a bit overbearing).

Commit that introduced spec for reference: ccb88bb

[ksylvest]

@HazAT do you know if anyone on the sentry team is available to help with issues / PRs?

[HazAT]

Hey, sorry about that.
Currently, we have no Ruby expert on the team, it's mostly me trying to keep up with everything that's going on.
So if you are interested in helping out, we are always looking for contractors, if you are interested, shoot me a mail my first name @ sentry.io :)

WRT to this PR, I would say #2 is fine.

[ksylvest]

Adjusted this PR to use no. 2.