Make TraceUtil support versions of cascading older than 2.6

[isnotinvain]

This was the original intention, but unfortunately it still triggers trying to load a class that doesn't exist pre 2.6

[isnotinvain]

The logs look like this:

15/01/29 23:42:04 WARN scalding.Tracing$: There was an error initializing tracing. Tracing information in DocumentServices such as Driven may point to Scalding code instead of user code. The most likely cause is a mismatch in Cascading library version. Upgrading the Cascading library to at least 2.6 should fix this issue.The cause was [java.lang.ClassNotFoundException: cascading.util.TraceUtil]

So as written, we only get the message of the exception not the full trace. Normally I'd update this to properly log the full stack trace, but given that this message is going to happen a lot / the full trace probably isn't that useful, maybe we should leave it as is?

[ianoc]

LGTM merge when green