Fix memory leaks; fix travis file; linux build

[tdeleon]

• The URLSession object in CKURLRequest was not calling finishTasksAndInvalidate(), which causes a memory leak. URLSession holds a strong reference to the delegate, and if it's not cleaned up then each session instance stays around forever. From the docs:

Important The session object keeps a strong reference to the delegate until your app exits or explicitly invalidates the session. If you don’t invalidate the session, your app leaks memory until it exits.

• Operations had retain cycles in their completion closures (missing [weak self] capture list)
• Fix travis file
• Build on linux

[BennyKJohnson]

Thanks for your improvements