Update composer.json

Add missing sqlite3 extension to require-dev as it is needed for the unit tests.

composer.json

@@ -69,6 +69,7 @@
     },
     "require-dev": {
         "ext-curl": "*",
+        "ext-sqlite3": "*",
         "mikey179/vfsstream": "~1.6",
         "simplesamlphp/simplesamlphp-module-adfs": "dev-master",
         "simplesamlphp/simplesamlphp-test-framework": "^1.1.3",