(doc) Update README to link to Apache HttpAsyncClient
(doc) Update README to link to Apache HttpAsyncClient

Previously, this library was a wrapper around http-kit, but has since been
updated to use the Apache HttpAsyncClient library instead. This commit updates
the README to link to HttpAsyncClient rather than http-kit.

puppetlabs/clj-http-client,camlow325/clj-http-client,dmcpl/clj-http-client,rlinehan/clj-http-client,puppetlabs/clj-http-client,scottyw/clj-http-client,rlinehan/clj-http-client,camlow325/clj-http-client,dmcpl/clj-http-client,scottyw/clj-http-client