Bug 1069545: Add setting to pin OSX downloads.

We have to await approval before offering new versions
of Firefox for download to OSX until the release of Fx 34.

With this we'll have to manually update the settings file when
we get approval, but at least it's in one spot.
Bug 1069545: Add 'osx_is_behind' to firefox/new/ page context.
Merge pull request #2314 from pmclanahan/pin-osx-download-to-specific-version

Bug 1069545: Add setting to pin OSX downloads.