feat(lib,test,ci):drop support of Node <=6.x and use async function

[qingdengyue]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

1.Drop support of Node <=6.x
2.Change generator to async function
3.CI remove Node 6 support

[codecov]

Codecov Report

Merging #37 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files           6        6           
  Lines         210      210           
=======================================
  Hits          207      207           
  Misses          3        3

Impacted Files	Coverage Δ
lib/helper.js	100% <100%> (ø)	⬆️
lib/cmd/start.js	99.18% <100%> (ø)	⬆️
lib/cmd/stop.js	93.54% <100%> (ø)	⬆️

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[atian25]

thanks for your work, but I don't think it's the right time to release a major version.

it's better to wait for eggjs/egg#3644 (Better Single Process / CLI Plugin System) and see what's need then release one major version together.

so could we just keep this PR for a while? thanks for your works again.

[qingdengyue]

@atian25 ok.thanks.

[fengmk2]

#58