Upgrade to cheshire 5.2.0 and removal of cheshire.custom as it's deprecated

Since cheshire.custom has been deprected in favour of cheshire.json and cheshire.generate (http://dakrone.github.io/cheshire/cheshire.custom.html), I updated cheshire in this library, as well as cloureworkz.support and upgrade clj-http to a version that supported cheshire 5.2.0

All the rest tests look to pass for me.

Related to:
https://github.com/clojurewerkz/support/pull/1