Uses of tracing functions leave warnings

[yairchu]

This way you can use traces for debugging but have warnings to help you remember to remove them and not leave stale traces in your code.

[snoyberg]

Yes, please update the version numbers on the -conduit and -yesod packages too, and tighten the lower bounds. We want the version of the -conduit and -yesod packages to determine the minor version of classy-prelude.

[yairchu]

Ok, good now?

[snoyberg]

Thanks!