Update analysis results file version to 2.0

We added a new field to the analysis results file format in legacy/trac#26673 (moved), but we did not yet increment the format version number. The current version field is a floating point number, which doesn't work well for versioning. We should use a version string here. Given that this change alone is going to be backward-incompatible, we'll have to call the new version 2.0. I'm preparing a patch.