Fixing RACDynamicSignal.h import

After wrestling with ReactiveCocoa and CocoaPods for a while I discovered that `RACDynamicSignal.h` imported its super via `#import <ReactiveCocoa/ReactiveCocoa.h>` instead of just the necessary `#import RACSignal.h`,  This caused a ton of redefinition errors when installing ReactiveCocoa via CocoaPods (see image).

This change seems to fix the problem allowing it to compile successfully. :pray: 

![image](https://f.cloud.github.com/assets/130534/1950411/a9061436-8145-11e3-939e-2e885bdbf867.png)