Allow sonata-admin 2.3

See https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/270
is sonata admin now following semver? or does 2.3 still only mean its compatible with symfony 2.3 but BC breaks might happen in the middle of 2.3.x? this was the reason we did no depend on 2.2.* but limited to the last tested version.

/cc @rande