Breakage related to http-client-0.5.0 / http-conduit-2.2.0

[sjakobi]

I've had a Travis job fail because it picked a new version of http-conduit:

https://travis-ci.org/sjakobi/stack/jobs/142306535#L1131

I guess we should just adapt the code and add lower bounds for http-client and http-conduit.