Add React compatibility layer

This suppresses warnings for projects which do not include PropTypes or versions of React less than 15.5.

src/create-glamorous.js

@@ -1,4 +1,4 @@
-import PropTypes from 'prop-types';
+import PropTypes from './react-compat';
 import React from 'react';
 import {StyleSheet} from 'react-native'
 import {CHANNEL} from './constants'

src/react-compat.js

@@ -0,0 +1,15 @@
+import React from 'react'
+
+// eslint-disable-next-line import/no-mutable-exports
+let PropTypes
+
+if (parseFloat(React.version.slice(0, 4)) >= 15.5) {
+  try {
+    PropTypes = require('prop-types')
+  } catch (error) {
+    // ignore
+  }
+}
+PropTypes = PropTypes || React.PropTypes
+
+export default PropTypes

src/theme-provider.js

@@ -1,4 +1,4 @@
-import PropTypes from 'prop-types';
+import PropTypes from './react-compat';
 import React from 'react';
 import brcast from 'brcast'
 import {CHANNEL} from './constants'

src/with-theme.js

@@ -1,4 +1,4 @@
-import PropTypes from 'prop-types';
+import PropTypes from './react-compat';
 import React from 'react';
 import {CHANNEL} from './constants'