Removed deprecated use of PropTypes in react error message. (#7)

package.json

@@ -38,6 +38,7 @@
     "node": ">= 5.11.0"
   },
   "dependencies": {
+    "prop-types": "^15.5.10",
     "react": "^15.3.2",
     "react-dom": "^15.3.2"
   }

src/Step.js

@@ -1,4 +1,5 @@
-import React, { Component, PropTypes } from 'react';
+import React, { Component } from 'react';
+import { PropTypes } from 'prop-types';
 
 export default class Step extends Component {
   constructor() {

src/Stepper.js

@@ -1,4 +1,5 @@
-import React, { PropTypes } from 'react';
+import React from 'react';
+import { PropTypes } from 'prop-types';
 
 import Step from './Step';