Install polymer-cli as first step of migrating from polylint

The official successor to polylint is bundled in the polymer command
line:

https://github.com/Polymer/polymer-linter

We can start using this today by adding polymer-cli to our set of
packages to npm install. Later on, we can remove polylint from this
same set, but for now there is no conflict.

Change-Id: I34712053b03296c6ec36f9a4a6ff8d7796b3362c
1 file changed
tree: ef887041af14f60a8ab842a7c2480186ebd6aae4
  1. jenkins/
  2. jenkins-docker/
  3. .gitignore