docs: pdate README.md to suggest --save-dev (#437)

There is very little chance people will need coverage as more then a dev dependency.

README.md

@@ -314,7 +314,7 @@ integrated with coveralls and travis-ci.org:
 1. add the coveralls and nyc dependencies to your module:
 
   ```shell
-  npm install coveralls nyc --save
+  npm install coveralls nyc --save-dev
   ```
 
 2. update the scripts in your package.json to include these bins: