Delete your keyboards, download app, re add keyboards
This is really annoying, and I don't know how or why this fixes the issue but at least you can do it all from your idevice.
Go to your settings and remove all your keyboards.
Quit out of app store and settings through multitasking.
Try to download the app again, it should work.
Then add your keyboards back.
If it doesn't work, try adding/deleting/adding/deleting extra keyboards a couple times. You might want to switch from English US to English UK.
After some puzzling around with it, you'll get the apps to download and you shouldn't have the problelm again (at least I haven't in a couple weeks).
Good luck