Update eslint-config npm install command (facebook#3072)

The `npm install` script for outside usage instructions is outdated. Updated to match the package's current peer dependencies.

packages/eslint-config-react-app/README.md

@@ -19,7 +19,7 @@ If you want to use this ESLint configuration in a project not built with Create
 First, install this package, ESLint and the necessary plugins.
 
   ```sh
-  npm install --save-dev eslint-config-react-app babel-eslint@7.2.3 eslint@3.19.0 eslint-plugin-flowtype@2.33.0 eslint-plugin-import@2.2.0 eslint-plugin-jsx-a11y@5.0.1 eslint-plugin-react@7.0.1
+  npm install --save-dev eslint-config-react-app babel-eslint@^7.2.3 eslint@^4.1.1 eslint-plugin-flowtype@^2.34.1 eslint-plugin-import@^2.6.0 eslint-plugin-jsx-a11y@^5.1.1 eslint-plugin-react@^7.1.0
   ```
 
 Then create a file named `.eslintrc` with following contents in the root folder of your project: