Pin JSON version to fix specs on ruby head

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' - pinning the version.

.travis.yml

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

sentry-raven.gemspec

@@ -18,4 +18,5 @@ Gem::Specification.new do |gem|
   gem.executables = "raven"
 
   gem.add_dependency "faraday", ">= 1.0"
+  gem.add_dependency "json", "<= 2.2.0"
 end