Update to psr/http-message 0.3.0, phly/http 0.5.0

This changeset updates Conduit to psr/http-message 0.3.0 and phly/http 0.5.0.
The primary changes include:

- Typehinting against `Psr\Http\Message\IncomingRequestInterface` instead of
  `Psr\Http\Message\RequestInterface`. This should not affect existing
  consumers, unless they are extending `Phly\Conduit\Middleware` and overriding
  the `__invoke()` method.
- Adding `setProtocolVersion()` to each of `Phly\Conduit\Http\Request` and
  `Phly\Conduit\Http\Response`.
- Adding the new methods defined in `Psr\Http\Message\IncomingRequestInterface`
  to `Phly\Conduit\Http\Request`.

Documentation and tests have all been updated to reflect the above changes.
Merge branch 'feature/http-message-0.3.0'

Close #7
[0.6.0] release readiness