Auto-truncate quickfix file on clean build

With reference to: https://github.com/dscleaver/sbt-quickfix/issues/10

I understood why the quickfix doesn't get truncated sometimes: it has to do with the fact that clean builds are cached and fixing issues can get you in a run where the "Compiling" message is not thrown in the logs. Using the debug logs I could find an invariant message that I could then use to reset the quickfix before running compilation in any case.

Please let me know if you find this useful!