clarify constraint for the Asset component

The conflict rule already forbids installing releases before 3.3 of the
Asset component. This will bring the constraint in the `require-dev`
section inline with the conflict rule.

src/Symfony/Bundle/FrameworkBundle/composer.json

@@ -35,7 +35,7 @@
     },
     "require-dev": {
         "fig/link-util": "^1.0",
-        "symfony/asset": "~2.8|~3.0",
+        "symfony/asset": "~3.3",
         "symfony/browser-kit": "~2.8|~3.0",
         "symfony/console": "~3.3",
         "symfony/css-selector": "~2.8|~3.0",