Use PSR-15 middleware standard

This finally adopts the new standardized interfaces instead of the
work-in-progress ones with the `Interop\` prefix.

Since we have now updated to PHP 7.1, we can also use Stratigility
3.0 as the middleware dispatcher.

composer.json

@@ -47,6 +47,8 @@
         "nikic/fast-route": "^0.6",
         "oyejorge/less.php": "~1.5",
         "psr/http-message": "^1.0",
+        "psr/http-server-handler": "^1.0",
+        "psr/http-server-middleware": "^1.0",
         "symfony/config": "^3.3",
         "symfony/console": "^3.3",
         "symfony/http-foundation": "^3.3",
@@ -54,9 +56,8 @@
         "symfony/yaml": "^3.3",
         "s9e/text-formatter": "^0.8.1",
         "tobscure/json-api": "^0.3.0",
-        "zendframework/zend-diactoros": "^1.6",
-        "zendframework/zend-stratigility": "^2.2",
-        "http-interop/http-middleware": "^0.4.0"
+        "zendframework/zend-diactoros": "^1.7",
+        "zendframework/zend-stratigility": "^3.0"
     },
     "require-dev": {
         "mockery/mockery": "^0.9.4",