Set the right version number to the nightly windows installer

Right now, the nightly installer that comes out from jenkins says that it is version 2.99.91. But it is actually generated from the trunks of the projects, so it's a lot higher than that.

For the results of the weekly testing of that installer, it's important to record the right number. Also, the documentation on Moztrap assumes that we have higher numbers for recent versions. If we don't follow that convention, we'll have to update a lot of things manually.

Also, when we start testing the updates of released version to this nightly versions, the right version number must be in place.

<dobey> i think it has the same numbers in the xml though. we haven't fixed it to be more automagic yet, afaik
<briancurtin> we could make jenkins produce an installer file name with a new version or something if wanted, but the actual binaries will be produced with whatever the XML says
<dobey> right. we need to automate it a bit