Updated README
Updated README

... to remove API reference to `Phly\Conduit\Http`, as well as to reference
`Phly\Http` in all examples that reference request, response, and/or server
instances.

Additionally, updated the `composer require` statement to pull in the
`psr/http-message` and `phly/http` packages, as they are each at dev dependency.

liuggio/zend-stratigility,Maks3w/zend-stratigility,jguittard/zend-stratigility,xtreamwayz/zend-stratigility,codeliner/zend-stratigility,matwright/zend-stratigility,Diego-Brocanelli/zend-stratigility