upgrade superagent to 0.19.0

superagent 0.18.0 requires qs 0.6.6, this has a known vulnerability
(https://nodesecurity.io/advisories/qs_dos_extended_event_loop_blocking). superagent
0.19.0 updates qs to 1.2.0, fixing this vulnerability.
Merge pull request #166 from balaclark/update-superagent

upgrade superagent to 0.19.0