Support Unix sockets
Update version of Excon with new feature
Swap method declaration and if statement
Move legacy excon methods into a helper module.

By putting it in a separate file it's not loaded and
counted in the simplecov coverage calculation,
which matters since we're not running our build against
the old version of Excon.
Excon 0.28 works.
Changelog and README update for #345.
Be tolerant of ::Excon::Utils being undefined.

It was added in Excon 0.27.5, and we support
older excon versions than that.
Merge pull request #347 from vcr/improve-on-345

Improve on 345