Replaced `frame` calls for `bounds`.

With this changes it works properly on iOS 7 while compiling on Yosemite with iOS 8 Beta 5 SDK.
This is probably an iOS bug (I am not sure, though). In any case, calling bounds is better, in my point of view, because it refers to the view's own coordinate system. `frame` should only be used when manipulating multiple other sibling views at the same time.