Fix for last version of symfony

[benji07]

when i update symfony this bundle doesn't work anymore.

[Symfony\Component\Config\Definition\Exception\InvalidDefinitionException]

Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition::useAttributeAsKey() is not applicable to concrete nodes.

[stof]

Please wait a bit before merging this. The change in Symfony will probably be reverted as @schmittjoh said it is wrong

[lsmith77]

@stof: is this issue tracked in a ticket or a PR?

[stof]

Looking at it again, using this method is indeed wrong here. Symfony 2.0 simply ignored it but removing it is indeed a good idea so you can merge it.
@schmittjoh said it is wrong in the PR sent by @vicb but he has not explained why when asking for details

[schmittjoh]

What is wrong is the Assetic PR as it is not behaviorally equivalent to the previous version. If the changes were necessitated by victor's PR on symfony (which I haven't had time to review yet), then there is something wrong as well.

[stof]

@schmittjoh the PR done by @vicb on symfony makes the validation stricter instead of silently ignoring methods that make no sense for prototyped nodes (similarly to some other methods that were already validated)