Strip trailing slash if present in apiURL: individual API calls duplicate it

Utterly trivial change: you can [blame](http://logbot.glob.com.au/?c=mozilla%23developers&s=6+Nov+2012&e=6+Nov+2012&h=#c426151) [edmorley](http://logbot.glob.com.au/?c=mozilla%23developers&s=11+Jan+2013&e=11+Jan+2013&h=#c487073) for this!