composer.json fixed for Symfony 2.3

[petk]

No description provided.

[bamarni]

this is not the correct fix, see doctrine/data-fixtures#103

[stof]

this will not fix it.
The issue you have with Symfony 2.3 is that you changed your minimum stability, and so you don't find data-fixtures anymore. But an explicit requirement of a dev version will not whitelist it outside of the root package, so it won't help you