Compatibility with GNOME 3.4

Hi,
The clock actor seems to be automagicaly added since GNOME 3.4. So, no need to add it by hand (anyway, it failed to add). It will probably break the compatibility with GNOME≤3.2, though… I don't know if there is a way to check for g-s version from an extension… (I never took a look at extensions before this one).

Anyway, do whatever you want with this, it's just a hint for what don't work with g-s 3.4.