Update dependency superagent to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
superagent	devDependencies	major	5.3.1 -> 6.0.0

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Release Notes

visionmedia/superagent

v6.0.0

Compare Source

• fix: bump deps, fixed linting, specific retry approach d68907b
• fix: fixed content-type case-sensitivity (closes #​1572) 44fadf9
• Add mention of bearer auth to docs (#​1583) e88a13f
• Update jQuery in tail.html (#​1582) a009ace
• Revert "Improve .retry() method functionality with delays and prevent calling on successful request (#​1527)" 74641bb
• fix: fixed lint, yarnlock 0c4f96f
• Improve .retry() method functionality with delays and prevent calling on successful request (#​1527) 62eae78
• fix: fixed travis f8ea331
• removed extra should from test description (#​1581) cc34638

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[coveralls]

Coverage remained the same at 89.8% when pulling 1cc2022 on renovate/superagent-6.x into 55d5071 on master.