Update README

- Removes references to `Phly\Http\ResponseInterface` (no longer exists)
- Updates the version for `psr/http-message` to `~0.2.0@dev`
- Adds descriptions for `Phly\Conduit\Http\Request` and
  `Phly\Conduit\Http\Response`.
Updated changelog
Merge branch 'hotfix/update-readme'

Update readme to remove references to obsolete/removed functionality.