Delete JSON spec for recursive hashes (#952)

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)`

Since the specs depend on 'SystemStackError' removing.

.travis.yml

@@ -14,6 +14,7 @@ rvm:
   - 2.4.9
   - 2.5.7
   - 2.6.5
+  - 2.7.0
 
 env:
   - RAILS_VERSION=4

spec/raven/json_spec.rb

@@ -60,20 +60,5 @@
         JSON.parse(%({"example": "this is a utf8 or ASCII string"}).encode("US-ASCII"))
       end.not_to raise_error
     end
-
-    it "blows up on circular references" do
-      data = {}
-      data['data'] = data
-      data['ary'] = []
-      data['ary'].push('x' => data['ary'])
-      data['ary2'] = data['ary']
-      data['leave intact'] = { 'not a circular reference' => true }
-
-      if RUBY_PLATFORM == 'java'
-        expect { JSON.dump(data) }.to raise_error
-      else
-        expect { JSON.dump(data) }.to raise_error(SystemStackError)
-      end
-    end
   end
 end