AppStore won't install OS X 10.10.5 update
I updated from 10.10.4 and had the very same problem. Even after rebooting this loop did not end. I solved it by installing the update from the command line with this command:

softwareupdate -i -a

In order to issue this command, open the Terminal application and enter the command. The tool downloaded the software update package and installed it. At the end the tool notifies you to restart your system. After the restart 10.10.5 was installed and everything was fine.

For details about this tool see https://support.apple.com/en-us/HT200113