Fix: Switch to PSR-4 autoloading

composer.json

@@ -31,6 +31,13 @@
         }
     },
     "autoload": {
-        "psr-0": { "Solarium\\": "library/" }
+        "psr-4": {
+            "Solarium\\": "library/Solarium/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Solarium\\Tests\\": "tests/Solarium/Tests/"
+        }
     }
 }