NASA’s climate data (text file) shows August 2015 as the second-warmest August on record, very close to 2014’s record (with 2011 in third place). According to the Japan Meteorological Agency, August 2015 was the hottest August on record, with 2014 in second place, followed by 1998.
(Their numbers are different because they’re using different data sets. Both show a steady warming trend since the 1890s.)
The year-to-date averages also show that this January-through-August is the hottest eight-month start to any year on record, already ahead of 2014’s record-setting pace.