Drop node engine down to 6.9.0 to support AWS lambda environments.

[thomasmichaelwallace]

Drops node engine version down, and confirms build/test against node 6.9.0.

This resolves issue #141, which in-turn will enable webpack/webpack#6579 to be resolved.

[jsf-clabot]

All committers have signed the CLA.

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[codecov]

Codecov Report

Merging #142 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #142   +/-   ##
=======================================
  Coverage   92.57%   92.57%           
=======================================
  Files          37       37           
  Lines        1454     1454           
  Branches      336      336           
=======================================
  Hits         1346     1346           
  Misses        108      108

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[ooflorent]

Thanks!