don't require Windows daily builds to have '.installer' in their names

@whimboo: This is the separate pull request that reverts a change you made in [pull 31](https://github.com/mozilla/mozdownload/pull/31) that broke downloading of Windows B2G builds, which don't have an installer variant.

See for example this directory containing the most recent Windows build:

https://ftp.mozilla.org/pub/mozilla.org/b2g/nightly/2012/12/2012-12-13-09-53-19-mozilla-b2g18/

Presumably you made the change for a reason, though, and I don't want to break that other use case! Perhaps you can elaborate, and then we can figure out how to keep both working?