Filter prerelease builds by product name as well.

Sometimes "--product b2g" would return a firefox build that lived in the same
directory if its timestamp was as-recent/more-recent than the b2g builds that
live in the directory.

See https://github.com/mozilla-b2g/mozilla-download/issues/8 for more context.
Merge pull request #12 from asutherland/filter-by-product

Filter prerelease builds by product name as well.