Explicitly cast int to float in PMA_Util::formatByteDown()

Signed-off-by: Jeff Chan <jefftchan@gmail.com>

Same PR as #910, now with signed off message.

**Problem:** Zend PHP interprets integers as floats if it encounters an integer overflow. HHVM, however, does not, and thus causes an overflow.

**The Fix:** Explicitly convert integer into float