Add the exact versions to the vulcanize.md install command.

docs/vulcanize.md

 # Vulcanizing Chrome Polymer UIs
 
 `vulcanize` is an npm module used to combine resources.  In order to make the
 Material Design downloads and history pages sufficiently fast, we run vulcanize
 on the source files to combine them and reduce blocking load/parse time.
 
 ## Required software
 
 Vulcanization currently requires:
 
@@ skipping 11 matching lines @@
 
 We recommend telling npm where to store downloaded modules:
 
 ```bash
 $ npm config set -g prefix "$HOME/node_modules"
 ```
 
 Then install the required modules:
 
 ```bash
-$ npm install -g crisper vulcanize uglifyjs polymer-css-build
+$ npm install -g crisper@2.0.1 vulcanize@1.14.8 uglifyjs@2.4.10 \
+    polymer-css-build@0.0.6
 ```
 
 Ultimately, all that is required to run this script is that the node binaries
 listed above are on your $PATH.
 
 ## Combining resources with vulcanize
 
 To combine all the CSS/HTML/JS for all pages which use vulcanize, making them
 production fast, you can run the command:
 
 ```bash
 $ chrome/browser/resources/vulcanize.py  # from src/
 ```
 
 This should overwrite the following files:
 
 - chrome/browser/resources/md_downloads/
  - vulcanized.html (all <link rel=import> and stylesheets inlined)
  - crisper.js (all JavaScript, extracted from vulcanized.html)
 - chrome/browser/resources/md_history/
  - app.vulcanized.html
  - app.crisper.js
 
 ## Testing downloads without vulcanizing
 
 Build with `use_vulcanize = false` in your gn args to build without vulcanizing.