Fix comparison of frozen sources.

`v[0] == v[1]` is no longer appropriate with source blocks, because the dependency source may be a pinned source, but the lockfile source (because we couldn't change the format) will be the aggregate Rubygems source for all gems that came from a gem server.

Note that with our current design, the lock file does not change if you switch gem servers for a gem in the Gemfile after freezing. We have no way to detect a gem server change.