Make sure that default channel will follow nightly

Fixes [bug 914533](https://bugzilla.mozilla.org/show_bug.cgi?id=914533). Probably too hackish way to achieve this but I'd like some input since I'm new to the snippets project :)

You guys use [lolcommits](http://mroth.github.io/lolcommits/)? Because I use lolcommits a lot!
![31214318ef9](https://cloud.githubusercontent.com/assets/318208/4911168/b351ba12-6489-11e4-950e-aa42894efab1.gif)

I actually don't think we should bother altering how client match rules are evaluated, as someday we might want a CMR that only matches the `default` channel. Changing how the `match_client` method works above is enough.