override getModifiedTime for Cascading >= 2.1

[sorenmacbeth]

The default SkipStrategy in Cascading 2.1 and greater uses the timestamp of the directory returned by getPath(), which in the case of a Pail doesn't change. We need to override getModifiedTime to return 0L when used as a sink and something else (currentTime maybe) when used as a source.

http://docs.cascading.org/cascading/2.1/userguide/html/ch03s08.html#N20CA7