Fix linker error with ProximityKit

The framework's binary library was not being properly found by Xcode, added a framework search path to the spec to resolve.


```
% pod spec lint 0.2.6/ProximityKit.podspec 

 -> ProximityKit (0.2.6)

Analyzed 1 podspec.

ProximityKit.podspec passed validation.

% pod spec lint 0.2.3/ProximityKit.podspec

 -> ProximityKit (0.2.3)

Analyzed 1 podspec.

ProximityKit.podspec passed validation.
```