Explicitly support the latest Excon release.
Ensure Excon responses get recorded when :expects middleware raises an error.
Set minimum excon version to 0.25.2.

That's the version where Excon::Middleware::ResponseParser
was introduced.
Don't allow excon to mutate our stubbed response.

This causes problems when the response is written
back to disk (e.g. when `:new_episodes` is used).
Merge pull request #356 from vcr/fix-excon-idempotent-retries

Fix for excon idempotent retries