fixed Xcode 8.3 warning (#40)
fixed Xcode 8.3 warning (#40)

Fixed Xcode 8.3 warnings:

```
./RMStepsController/RMStepsController/RMStepsBar.m:116:37: warning:
assigning to 'id<CAAnimationDelegate> _Nullable' from incompatible type
'RMStepSeperatorView *const __strong'
        fillColorAnimation.delegate = self;
                                    ^ ~~~~
./RMStepsController/RMStepsController/RMStepsBar.m:133:37: warning:
assigning to 'id<CAAnimationDelegate> _Nullable' from incompatible type
'RMStepSeperatorView *const __strong'
        fillColorAnimation.delegate = self;
                                    ^ ~~~~
```

CooperRS/RMStepsController