Torbutton should fetch a version list from check.tp.o
We hacked check.tp.o to return a minimal XHTML response to https://check.torproject.org/?TorButton=true.
We should create a new parameter that returns a version list of recommended Tor Browser Bundle versions at a url like https://check.torproject.org/?ReportVersions=true
Torbutton should check to see if the current TBB version is in this list and if is not, it should use a yellow-themed version of check.torproject.org that recommends that the user upgrades.