chore: add karma-script-launcher plugin

This plugin is shipped as a default one with Karma. It's specified as a peer dependency.

I assume, there's an old version of NPM on the CI server, which does not support peerDependencies and therefore it didn't get installed.

This will make the dependency explicit.

package.json

@@ -21,6 +21,7 @@
     "karma-ng-scenario": "~0.0.1",
     "karma-junit-reporter": "~0.0.1",
     "karma-sauce-launcher": "~0.0.4",
+    "karma-script-launcher": "~0.0.1",
     "yaml-js": "0.0.5",
     "showdown": "0.3.1"
   },