chore(docs): Remove plugins property from example (#160)

karma loads all plugins with 'karma-' automatically, including 'plugins' property in config leads to cryptic error messages when attempting to run karma.

README.md

@@ -70,11 +70,7 @@ module.exports = function(config) {
       // webpack-dev-middleware configuration
       // i. e.
       stats: 'errors-only'
-    },
-
-    plugins: [
-      require("karma-webpack")
-    ]
+    }
   });
 };
 ```