Update superagent version due to sec vulnerability

[ttrabold]

The used superagent version has a Large gzip Denial of Service vulnerability. See: https://nodesecurity.io/advisories/superagent_large-gzip-denial-of-service

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[ttrabold]

I signed it!

[googlebot]

CLAs look good, thanks!

[mastermatt]

https://nodesecurity.io/advisories/479 can be removed from .nsprc along with this patch.

[erathinam]

Will this be merged ?? The current stable version of superagent is 5.1.0