0.5.0: Merge pull request #21 from richardbarrell-calvium/proptypes-fix

This release makes react-native-component-viewer compatible with react-native 0.49.1.

Unfortunately, the fix for that means it loses support for react-native older than 0.43.0. For older versions of RN, please use version 0.4.0 or below of react-native-component-viewer.