Improve constraints of fat composer.json

* Change symfony/* from 2.1.* => ~2.1
* Delegate twig and monolog version selection to bridges

bin/skeleton/fat_composer.json

 {
     "require": {
         "silex/silex": "1.0.*@dev",
-        "symfony/browser-kit": "2.1.*",
-        "symfony/console": "2.1.*",
-        "symfony/config": "2.1.*",
-        "symfony/css-selector": "2.1.*",
-        "symfony/dom-crawler": "2.1.*",
-        "symfony/filesystem": "2.1.*",
-        "symfony/finder": "2.1.*",
-        "symfony/form": "2.1.*",
-        "symfony/locale": "2.1.*",
-        "symfony/process": "2.1.*",
-        "symfony/security": "2.1.*",
-        "symfony/serializer": "2.1.*",
-        "symfony/translation": "2.1.*",
-        "symfony/validator": "2.1.*",
-        "symfony/monolog-bridge": "2.1.*",
-        "symfony/twig-bridge": "2.1.*",
-        "monolog/monolog": ">=1.0.0,<1.2-dev",
-        "twig/twig": ">=1.2.0,<2.0-dev",
+        "symfony/browser-kit": "~2.1",
+        "symfony/console": "~2.1",
+        "symfony/config": "~2.1",
+        "symfony/css-selector": "~2.1",
+        "symfony/dom-crawler": "~2.1",
+        "symfony/filesystem": "~2.1",
+        "symfony/finder": "~2.1",
+        "symfony/form": "~2.1",
+        "symfony/locale": "~2.1",
+        "symfony/process": "~2.1",
+        "symfony/security": "~2.1",
+        "symfony/serializer": "~2.1",
+        "symfony/translation": "~2.1",
+        "symfony/validator": "~2.1",
+        "symfony/monolog-bridge": "~2.1",
+        "symfony/twig-bridge": "~2.1",
         "doctrine/dbal": ">=2.2.0,<2.4.0-dev",
         "swiftmailer/swiftmailer": "4.1.*"
     }