Update to psr/http-message 0.4.0 && phly/http 0.6.0

Updates to psr/http-message 0.4.0 and phly/http 0.6.0. These libraries had several BC incompatible changes, requiring changes in Conduit:

- Removal of `setHeaders()` and `addHeaders()` in both the request and response decorators.
- Renaming of `(set|get)PathParams()` to `(set|get)Attributes()` in the request decorator.
- Specifying `array` as the only accepted input type and return type for all `IncomingRequestInterface`-specific methods.