Trying to install Xcode on Yosemite 10.10.5
You are trying to install Xcode 7 on OS X 10.10. There's your problem. If you read the developer website, it would tell you it can only be installed on OS X 10.11. No exceptions. My advice would be to either: a) use a new computer to test Xcode 7 and El Capitan, b) Use a new partition for that, or c) Wait for the official releases within a month or so.