Use npm bin when calling gulp

[jjinux]

Otherwise, you'll get "-bash: gulp: command not found" when gulp is not
installed globally.

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[jjinux]

I signed it!

[googlebot]

CLAs look good, thanks!

[jjinux]

Hey, @jeffposnick, are you going to merge this PR so that I can delete my fork and local repo? ;)

[jeffposnick]

Sorry, @jjinux, I lost track of this over the break. I forget that not everyone already has a global gulp installed, so sure, this LGTM.

[jjinux]

@jeffposnick, thanks!