use pdo_sqlite rather than sqlite3 extension

changed the extension on the request of @thijskh

composer.json

@@ -28,7 +28,7 @@
         "whitehat101/apr1-md5": "~1.0"
     },
     "require-dev": {
-        "ext-sqlite3": "*",
+        "ext-pdo_sqlite": "*",
         "phpunit/phpunit": "~3.7",
         "satooshi/php-coveralls": "^1.0"
     },