npm version must be >=2.0.0

In order to support passing arguments to npm, you need to be running npm version 2.0.0 or greater:
http://blog.npmjs.org/post/98131109725/npm200

Update instructions with how to check and how to update npm.

tests/README.md

@@ -14,7 +14,7 @@ The TodoMVC project has a great many implementations of exactly the same app usi
 
 ## Running the tests
 
-These tests use Selenium 2 (WebDriver), via the JavaScript API (WebdriverJS).  In order to run the tests you will need to install the dependencies. Run the following command from within the `tests` folder:
+These tests use Selenium 2 (WebDriver), via the JavaScript API (WebdriverJS).  In order to run the tests you will need to install the dependencies.  NPM must be version 2.0.0 or greater, so upgrade it first with `npm install -g npm` if `npm --version` outputs anything less than 2.0.0.  Run the following command from within the `tests` folder:
 
 ```sh
 $ npm install