Fix edd-reports page not working on HHVM

This fixes [the issue we discussed on Twitter](https://twitter.com/pippinsplugins/status/512231253832400897) yesterday and gets the Reports > Earnings page to work again.

Note that `CAL_GREGORIAN` is still undefined on HHVM, could be an idea to just define that to `0` if it  isn't defined but I am not sure if that is the best way to go about this.