GraphQL 1.5.3 contains an important security fix for 1.5.x, 1.4.x is not vulnerable

Hi! This weekend I discovered a DoS vulnerability in version 1.5.x. You should bump to 1.5.3 or revert to 1.4.x. Full details on this GitHub issue: 

https://github.com/rmosolgo/graphql-ruby/issues/620

- Robert