Fix for newer version of cereal

This package will not compile with version 4.0.0.0 of cereal - because the `Fail` constructor returned from `runGetPartial` has been changed. I tweaked the one place where this is used to ignore the extra field (which afaik is the remaining input which has not been parsed). I also updated the cabal file to depend on version 4.0.0.0.

As an alternative to this change, you could instead simply change the cabal file to require cereal < 0.4.0.0.

I have not tested this change, as there appear to be no test suites in the package.