iOS 8.4 app download problem
I had this problem with Pages. I made a backup of my iPad to iTunes (just in case) and then I deleted the apps from the iPad through iTunes, then I clicked Sync and the apps were gone, then I just reinstalled them through the App Store and they worked again. Strangely, this happened again, but with Pages and Numbers this time and I didn't have enough space on my MacBook to back my iPad up again, so I gave up. Fortunately, when I installed the iOS 9 Public Beta from Apple, both apps were fixed. iOS 9 Public Beta 2 is pretty buggy, so update at your own risk. You may just want to wait until iOS 9 comes out in the fall (probably late-September) if the app is not too important.