Remove ringcentral/psr7 because it appears to be abandoned and upstream projects would like to support psr/http-message ^2.0

[kwhat]

It is unclear if we need to wait for all of the headers before parsing or if by removing the PSR7 requirement we can simply just parse the first line of the http response. I left the strpos() in there in case there was a reason the code was waiting.

This resolves issue #64