Add in gnome2-utils gnome2_icon_cache_update...

...to ensure the sublime icon shows up in gnome immediately.

At the moment, when sublime-text installs, it add icons to /usr/share/icons/hicolor, but doesn't update the icon cache, meaning gnome will display it with the default application icon until another package (or the user) runs the gtk-update-icon-cache tool on the hicolor directory.  This adds the appropriate calls from gnome2-utils to do that automatically at package install time, so users will get the correct icon immediately.