Add VCR support to record http interactions.
Add VCR spec for 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.
Fix Polisher::Git::Pkg#build_srpm making a GET to rubygems.org.

This spec is heavily stubbed and this fix is less than ideal but providing a path to the Gem.new allows us to not download the .gem file and provide the path to that file.
Force all http connections in specs to require VCR cassettes.
Merge pull request #115 from jrafanie/add_vcr_testing_to_fix_koji_diff

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