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.