Add vcr testing to fix koji diff not using latest package version

Adding vcr enables testing and fixing a bug in Koji.diff:

Koji.diff should show the "latest" rpm version for each tag.
Previously, the f19-ruby tag had rubygem-sinatra packages for 1.3.4 and 1.3.5 versions.
Koji.diff reported 1.3.4 for the f19-ruby tag while most users would expect 1.3.5.
This line goes away if #114 is merged.