Update fmt from 5.3.0 to 6.1.1
Update fmt from 5.3.0 to 6.1.1

Summary: Updated fmt version to be on par with buck build. It was causing inconsistencies.

Reviewed By: vitaut

Differential Revision: D20528011

fbshipit-source-id: d9e04ed2c28b839eaeff24120162c4db4732fa55

rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp