Possibly replace httpbin remote APIs with webfakes::httpbin_app

[sckott]

via #286

In the long run, it would be better to switch to webfakes::httpbin_app(), but this will require some work because webfakes doesn't implement exactly the interface that vcr expects.

[hadley]

Mostly waiting on r-lib/webfakes#115

[sckott]

@hadley Should we bump this to the next milestone?

[hadley]

Yes I think so. It’s not urgent and there are only a few cases left.