Pin json version to < 2.10

Context:

Failure:
ActiveModelSerializers::Test::SchemaTest#test_that_raises_with_a_invalid_json_body [/home/runner/work/active_model_serializers/active_model_serializers/test/active_model_serializers/test/schema_test.rb:129]:
Expected /A JSON text must at least contain two octets!|unexpected token at ''/ to match "unexpected end of input at line 1 column 1".

bin/rails test /home/runner/work/active_model_serializers/active_model_serializers/test/active_model_serializers/test/schema_test.rb:119

Author: wasifhossain

Gemfile

@@ -82,6 +82,7 @@ group :test do
     end
   end
   gem 'codeclimate-test-reporter', require: false
+  gem 'json', '< 2.10', require: false
   gem 'm', '~> 1.5'
   gem 'pry', '>= 0.10'
   gem 'byebug', '~> 8.2' if RUBY_VERSION < '2.2'