Remove node 0.6 support

jsdom 0.8.4 is not compatible with node 0.6 anymore
https://github.com/tmpvar/jsdom/commit/ecdae9727dc13c486d39c0db446be93bde8d92d8
Saucelabs: reactivate safari and deactivate ff

Safari is working back on saucelabs
Firefox is too unreliable to be usable