PolyGerrit: Update docs on install nodejs 6
PolyGerrit: Update docs on install nodejs 6

nodejs 4 is no longer supported in polymer.

See https://github.com/Polymer/web-component-tester/blob/master/CHANGELOG.md#600---2017-05-15

Which is causing failures for ci. Lets update to nodejs 6.

Change-Id: Ie0e314785e5672e9f557e63c76616d33f601fe24
(cherry picked from commit 722987a7ab91cb6848348a1f83ba5068e05e44c3)

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