Add symfony/dependency-injection dependency

BaseFieldDescription::camelize() relies on this component.

In 2.3.3, the behaviour of Container::camelize() changed, meaning that 2.3.3 is the lowest supported version.

composer.json

@@ -28,6 +28,7 @@
         "symfony/class-loader":"~2.3",
         "sensio/generator-bundle": "~2.3",
         "symfony/translation":"~2.3",
+        "symfony/dependency-injection": "~2.3,>=2.3.3",
         "twig/twig": "~1.15",
         "twig/extensions": "~1.0",
         "sonata-project/exporter": "~1.0",