iOS 8.4 does not update apps
In addition to the answer of DDPWNAGE and user138479 you may try to update directly via the app site and click on "update" there instead of using the "update all" button in the update tab.

Click "App-Store" ➡️ "Updates" ➡️ Click on App Name ➡️ In App Details click on "Update"

Another option might be to check if there is another download hanging which blocks all of the other updates.

Click "App-Store" ➡️ "Updates" ➡️ "Purchased" ➡️ Check if any Apps have a running circle hence an update symbol and stop the update.

One more option is to connect your iPhone with a PC/MAC and switch it off. After that:
Press the "Power" and "Home" Button at the same time for exactly 10 seconds. The iphone switches into DFU mode and kills all the running processes. The display is supposed to be black. If the apple logo shows up a second time, youve pressed the buttons too long. 
After the 10 seconds, release the "Power" button but remain pressing the "Home" button. Your PC/MAC should recognize your iphone and probably install the DFU drivers. After that press the "Power" and "Home" buttons again for another 20 seconds, which causes the iphone to leave the DFU mode and to restart. 
Try again to update your Apps.

Those are the solutions I was able to find. Unfortunately for me none of them worked, but maybe you'll be successful.