feat: add Node 8 syntax support

[malept]

BREAKING CHANGE: Drops support for Node < 8.

• Convert to async/await syntax
• Adjust wrapError to be more async/await - friendly
• Drop glob-promise in favor of util.promisify
• Update dependencies which require Node >= 8

Closes #22.
Closes #24.
Closes #28.

[codecov]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #27   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines         190    196    +6     
=====================================
+ Hits          190    196    +6

Impacted Files	Coverage Δ
src/spawn.js	100% <100%> (ø)	⬆️
src/readelectronversion.js	100% <100%> (ø)	⬆️
src/readmetadata.js	100% <100%> (ø)	⬆️
src/desktop.js	100% <100%> (ø)	⬆️
src/template.js	100% <100%> (ø)	⬆️
src/sandboxhelper.js	100% <100%> (ø)	⬆️
src/error.js	100% <100%> (ø)	⬆️
src/installer.js	100% <100%> (ø)	⬆️

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[malept]

Seems good. I updated most of the modules that depend on it.