fix a crash with RubyMotion 2.0. use allocate instead of alloc

When use "initialize" as initializer, we should use "allocate" instead of "alloc".

if use "alloc" and use "init" in somewhere (https://github.com/sxross/MotionModel/blob/master/lib/motion_model/adapters/array_model_persistence.rb#L103), the object might be autoreleased unexpectedly.