Remove restrictions on update channels (#42)

This fixes #42 for the hotfix-1.5 branch. I tried first applying this to master, but there seem to be issues preventing me from running the update tests successfully (dependent packages not installed by setup.py, and a weird mozinstall error), I can't look into these right now but perhaps we can fix this for 1.5.x and leave the issue open for fixing in master?

Note that for some reason I am not being offered updates on the nightly-esr17 channel. I wonder if perhaps these updates are disabled for some reason? @ashughes1 do you know anything about this? I can replicate it manually too.