upgrade cascading to 2.0.7

A fix in cascading to 2.0.7 is necessary when using it in large clusters:

"Fixed issue where Hadoop forgets past job completion status of a job during very long running c.f.Flows and throws a NPE when queried for the result."

https://github.com/Cascading/cascading/blob/2.1/CHANGES.txt#L83

Scalding tests pass with 2.0.7, and ran a few actual jobs too.