title: Material UI Picker - date-fns issue
body: <p>I am using material UI in my react application and I am having difficulty getting the material-ui-pickers example to work.
  Here is the online example from Material UI.
  <a href="https://codesandbox.io/s/52p74362pl" rel="noreferrer">https://codesandbox.io/s/52p74362pl</a></p>
  
  <p>The error that i am getting is:</p>
  
  <pre><code> index.js:1446 ../node_modules/@date-io/date-fns/build/index.esm.js Module not found: `Can't resolve 'date-fns/addDays' in 'C:\Project4\CDE\IFS.CDE.Web\node_modules\@date-io\date-fns\build'`
  </code></pre>
  
  <p>here are my package.json files</p>
  
  <pre><code>{
    "name": "IFS.CDE.Web",
    "version": "0.1.0",
    "private": true,
    "dependencies": {
      "@material-ui/core": "^3.7.1",
      "@material-ui/icons": "^3.0.1",
      "ajv": "^6.5.2",
      "autosuggest-highlight": "^3.1.1",
      "axios": "^0.18.0",
      "classnames": "^2.2.5",
      "connected-react-router": "6.1.0",
      "date-fns": "^1.30.1",
      "downshift": "^2.0.10",
      "extract-svg-path": "^2.1.0",
      "file-saver": "^1.3.8",
      "install": "^0.12.1",
      "lodash": "^4.17.10",
      "material-ui-datetimepicker": "^1.0.7",
      "material-ui-image": "^3.0.3",
      "material-ui-pickers": "^2.1.1",
      "moment": "^2.23.0",
      "npm": "^6.5.0",
      "parse-svg-path": "^0.1.2",
      "prop-types": "^15.6.2",
      "rc-progress": "^2.2.6",
      "react": "^16.6.3",
      "react-autosuggest": "^9.3.4",
      "react-bingmaps": "^3.6.1",
      "react-color": "^2.14.1",
      "react-dnd": "^7.0.2",
      "react-dom": "^16.6.3",
      "react-dropzone": "^4.3.0",
      "react-file-viewer": "^0.5.0",
      "react-iframe": "^1.2.0",
      "react-loadable": "^5.5.0",
      "react-popper": "^0.10.1",
      "react-redux": "^6.0.0",
      "react-render-html": "^0.6.0",
      "react-router-dom": "^4.3.1",
      "react-scripts": "^2.1.2",
      "react-select": "^2.0.0",
      "redux": "^4.0.1",
      "redux-persist": "^5.10.0",
      "redux-thunk": "^2.3.0",
      "rimraf": "^2.6.2",
      "typeface-roboto": "^0.0.54",
      "uuid": "^3.3.2",
      "video-react": "^0.13.1"
    },
    "scripts": {
      "start": "rimraf ./build &amp;&amp; react-scripts start",
      "build": "react-scripts build",
      "test": "react-scripts test --env=jsdom",
      "eject": "react-scripts eject",
      "set-env": "set NODE_ENV=production"
    },
    "devDependencies": {},
    "browserslist": [
      "&gt;0.2%",
      "not dead",
      "not ie &lt;= 11",
      "not op_mini all"
    ]
  }
  </code></pre>
  
  <p>I have installed date-fns but i still get the error.
  <a href="https://www.npmjs.com/package/date-fns" rel="noreferrer">https://www.npmjs.com/package/date-fns</a>.
  I also deleted all the node module and did a npm install to see if that would solve the issue but it did not. </p>
  
  <p>Thanks for the help.</p>