Security Vulnerability: Update superagent Version to Fix!

[takwas]

There are known issues with jQuery versions before v3.5.0.

This resulted in the flagging of supergent versions prior to v6.0.0, where a fix has been applied.

Therefore, this library can be updated to use the latest version of superagent in order to fix vulnerability reports on this library based on its usage of superagent.

[austince]

I know we've been hesitant to upgrade superagent before (see #203) because we've contemplated dropping in favor of just fetch, though these efforts have stalled (my bad). That being the case, I would be on board with a PR upgrading superagent if there are not too many breaking changes. Is this something you would be interested in tackling?

[jakebrown58]

I see that super-agent was upgraded to 6.1, but that the library is still on 4.3.0, and thus when we do a fresh upgrade from the registry, it still pulls in the old version of 4.3.0 with super-agent 3.7.x, and the jQuery 3.2.1 which is flagged for security issues.