react and react-dom dependencies should have caret range

Created by: gaearon

I think https://github.com/facebookincubator/create-react-app/pull/1253 made it so that react and react-dom are also installed with exact range:

  "dependencies": {
    "react": "15.4.2",
    "react-dom": "15.4.2"
  },
  "devDependencies": {
    "react-scripts": "0.9.2"
  },

This is not right. We want to pin react-scripts but leave react and react-dom unpinned:

  "dependencies": {
    "react": "^15.4.2",
    "react-dom": "^15.4.2"
  },
  "devDependencies": {
    "react-scripts": "0.9.2"
  },

The easiest way to do it would probably be to remove the exact flag from the installation script in packages/create-react-app/index.js, and instead pin react-scripts to a specific version in the same function that moves react-scripts to devDependencies (also in that file).