Bump slimerjs version to 0.9.4

* update to [the latest stable version of SlimerJS](http://docs.slimerjs.org/0.9/release-notes.html#version-0-9-4)
* update tests
 * ``slimer.exit`` accepts an optional exit code but it is ignored because of a limitation in Firefox/XulRunner - [docs](http://docs.slimerjs.org/nightly/api/slimer.html#exit)
 * improve the test for version string
 * ``npm test`` now passes