When you update to Xcode7, you need to use the latest version of cocos 3.4.9. This is described here.
However, there has been a few reports of memory leaks. The problem is a mismatch, which arises if you update cocos2d in an existing SpriteBuilder project. Here is how to fix it.
In your project, search for NO_ARC. This will give you 3 hits in CCNoARC.m. Locate the first hit, saying
#define NO_ARC 0
If this setting is 0, you should make sure that the file CCNoARC.m in build settings, has no flags.
However, if the setting is non zero, CCNoARC.m should have the flag -fno-objc-arc.
As you can unload the workload of the CPU slightly by setting NO_ARC to 1, I suggest you use that setting if you are fiddling with it anyways.