update README.md on ubuntu and gcc version
update README.md on ubuntu and gcc version

Summary: it seems since D15286181, we updated docker os_image to ubuntu18 and gcc7. this diff changes the README.md to reflect the change.

Reviewed By: yfeldblum

Differential Revision: D15513580

fbshipit-source-id: 0a3518c21c912903921946850bd08c948f09c295

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