Bump required symfony/console version from 2.2 to 2.3

The current code uses Symfony Console's Command::getNativeDefinition method,
which is protected in 2.2, but public in 2.3 and newer. This was picked up by
the unit tests running against lowest deps:

https://travis-ci.org/stecman/symfony-console-completion/jobs/77916046

Author: stecman

composer.json

@@ -10,7 +10,7 @@
     ],
     "require": {
         "php": ">=5.3.2",
-        "symfony/console": "~2.2"
+        "symfony/console": "~2.3"
     },
     "require-dev": {
         "phpunit/phpunit": "~4.4"