chore: add license checking with js-green-license

[jinwoo]

Run jsgl on the repository as one of posttest checks.

[jinwoo]

I realize I have to npm publish js-green-licenses again to support Node 6 here. Not sure how not to run jsgl for Node 4 though.

[jinwoo]

Now using js-green-licenses 0.1.2. It supports Node 4 & 6.

[codecov-io]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #105   +/-   ##
=======================================
  Coverage   96.58%   96.58%           
=======================================
  Files          10       10           
  Lines         322      322           
  Branches       19       19           
=======================================
  Hits          311      311           
  Misses         11       11

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