Update Readme describing how to run headless tests locally.

Author: bentrm

README.md

@@ -136,13 +136,14 @@ prior to submitting your changes. Thanks.
 
 ## Headless tests with casperjs
 
-You can also run the above tests headlessly through casperjs:
+You can also run the above tests headlessly through casperjs.
 
 ```bash
+# Install casperjs via npm in the GeoExt 2 clone dir (only first time)
+npm install [email protected]
+
 # Run the suite in the root of the repository
-./tests/headless/casperjs-1.1-beta.1/bin/casperjs \
-  test \
-  tests/headless/run-testsuite.js
+./node_modules/casperjs/bin/casperjs test tests/headless/run-testsuite.js
 ```
 
 These tests are also run though [travis](https://travis-ci.org/geoext/geoext2).