Update CONTRIBUTING.md

Author: neilime

CONTRIBUTING.md

@@ -2,22 +2,7 @@
 
 ## RUNNING TESTS
 
-To run tests:
-
-- Make sure you have a recent version of PHPUnit installed; 4.8.0
-  minimally.
-- Enter the `tests/` subdirectory.
-- Execute PHPUnit
-
-  ```sh
-  % phpunit
-  ```
-
-- You may also provide the `--coverage-html` switch;
-
-  ```sh
-  % phpunit --coverage-html ./_report
-  ```
-
-  This will generate code-coverage report witch can be displayed in a browser by opening `tests/_report/index.html` file
+```bash
+$ composer test
+```