Switch Cascading dependencies in pom.xml to provided scope

[kkrugler]

Chris Severs had said:

More generally, I was thinking it might be nice to not force the include of cascading in cascading.avro. Does that sound reasonable to you (and anyone else)? Right now I think cascading.avro has a dependency on some version of cascading but it could be changed to be a provided dependency so it doesn't step on your chosen version of cascading if you use maven. It's also possible of course to exclude the cascading from cascading.avro in the pom file but it might be nice to do so by default.

And we only need to depend on cascading-hadoop (and cascading-local?), not cascading-core

[kkrugler]

Should be pretty easy to try out.