Update webmock and excon.

- Fix the way we make excon HTTP requests in our tests.
  The signature changed in https://github.com/geemus/excon/commit/e5b6bea83c18c13c7d3c9b2517bcc8aeb53f5f93
- Use new Excon middleware architecture to hook into the requests in a
  more robust way. @geemus wrote up a description of how they work here:
  https://github.com/vcr/vcr/issues/271#issuecomment-14449311
- Remove a couple of tests that were focused on edge-case bugs of the
  old Excon adapter approach. They are no longer valid tests for thew new
  middleware approach.

Closes #271 and #199.