Rename method to comply with Apple memory management rules

I renamed gradientWithColors to newGradientWithColors to comply with the naming conventions for memory management from Apple:

https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html#//apple_ref/doc/uid/20000994-BAJHFBGH

"You create an object using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy” (for example, alloc, newObject, or mutableCopy)."

This gets rid of the the warning you get when you go to Product -> analyze.