revert to using hardcoded beta path

ftp.mozilla uses inconsistent naming in its file paths. The alias
to latest-beta will result in errors when a new beta is released
(potentially daily) until puppet-firefox has been updated. This
resolves the issue by reverting the beta path so that it is pinned
to a specific version. We can rely on Firefox's internal updater
to keep things fresh in between train switch releases.
Merge pull request #21 from lonnen/use-most-stable-paths

revert to using hardcoded beta path