minor #918 Referencing scripts in composer.json to avoid duplication (1ed)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #918).

Discussion
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Referencing scripts in composer.json to avoid duplication

It's a new feature of composer ([referencing-scripts](https://getcomposer.org/doc/articles/scripts.md#referencing-scripts)) available as of [1.0.0-alpha11](https://github.com/composer/composer/releases/tag/1.0.0-alpha11).

Commits
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4935d2c Referencing scripts to avoid duplication

Author: fabpot

composer.json

@@ -23,21 +23,19 @@
         "symfony/phpunit-bridge": "~2.7"
     },
     "scripts": {
-        "post-install-cmd": [
+        "symfony-scripts": [
             "Incenteev\\ParameterHandler\\ScriptHandler::buildParameters",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::prepareDeploymentTarget"
         ],
+        "post-install-cmd": [
+            "@symfony-scripts"
+        ],
         "post-update-cmd": [
-            "Incenteev\\ParameterHandler\\ScriptHandler::buildParameters",
-            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
-            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
-            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
-            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
-            "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::prepareDeploymentTarget"
+            "@symfony-scripts"
         ]
     },
     "config": {