minor symfony#16773 [TwigBridge] Clean deps now that 2.8.0 is tagged (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[TwigBridge] Clean deps now that 2.8.0 is tagged

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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e6efe7e [TwigBridge] Clean deps now that 2.8.0 is tagged

Author: stof

src/Symfony/Bridge/Twig/composer.json

@@ -22,7 +22,7 @@
     "require-dev": {
         "symfony/asset": "~2.7",
         "symfony/finder": "~2.3",
-        "symfony/form": "~2.7.8|~2.8,>2.8-BETA1",
+        "symfony/form": "~2.7,>=2.7.8",
         "symfony/http-kernel": "~2.3",
         "symfony/intl": "~2.3",
         "symfony/routing": "~2.2",