Allow json gem versions at least 1.7 but less than 2.0
Allow json gem versions at least 1.7 but less than 2.0

jshorty/FuelSDK-Ruby,salesforce-marketingcloud/FuelSDK-Ruby,andersondias/FuelSDK-Ruby,Shopify/FuelSDK-Ruby,kev5873/FuelSDK-Ruby,salesforcefuel/FuelSDK-Ruby,Vestorly/FuelSDK-Ruby,Privy/FuelSDK-Ruby,ExactTarget/FuelSDK-Ruby