chore: Update install docs

Author: joshwiens

README.md

@@ -20,7 +20,7 @@
 
 ```bash
 # for webpack 2
-npm install --save-dev extract-text-webpack-plugin
+npm install --save-dev extract-text-webpack-plugin@2.1.2
 # for webpack 1
 npm install --save-dev [email protected]
 ```