Switch to exjsx

I was getting the following error from consul-ex using it with discovery (both head from github).

Last message: :timeout
State: %{em: #PID<0.362.0>, index: nil, service: "downloader", services: [], task: nil}
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in JSEX.Decoder.format_key/2
        (jsex) lib/jsex.ex:101: JSEX.Decoder.format_key("Node", {:config, :binary, false})
        (jsex) lib/jsex.ex:97: JSEX.Decoder.handle_event/2
        (jsx) src/jsx_decoder.erl:163: :jsx_decoder.string/5
        (jsex) lib/jsex.ex:17: JSEX.decode!/2
        (jsex) lib/jsex.ex:24: JSEX.decode/2
        (consul) lib/consul/request.ex:22: Consul.Request.process_response_body/1
        (consul) lib/consul/request.ex:8: Consul.Request.response/3
        (consul) lib/consul/health.ex:8: Consul.Health.req_get/3

The JSON from Consul appears correct (it parses properly) so I looked to upgrade jsex but it appears to have morphed into exjsx. So this patch switches consul-ex over to exjsx. This removed the error for me immediately and I've had no issues using discovery with this in place.