Doesn't work with React 16

When adding `import { GiftedForm } from 'react-native-gifted-form';` to a React 16 app you get an error

> undefined is not an object (evaluating 'React.PropTypes.string')

One would assume that this library needs to be updated to use the `prop-types` module instead https://www.npmjs.com/package/prop-types

Start at the GiftedForm entry point imported from react-native-gifted-form and search the package for React.PropTypes references. Reproduce the failure in a React 16 app, then verify that importing GiftedForm no longer raises the undefined React.PropTypes.string error and that the existing form behavior remains intact.

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