Use caret range in blog instructions

The release was a bit broken.

_posts/2017-04-07-react-v15.5.0.md

@@ -164,13 +164,13 @@ We recommend using [Yarn](https://yarnpkg.com/) or [npm](https://www.npmjs.com/)
 To install React with Yarn, run:
 
 ```bash
-yarn add react@15.5.0 react-dom@15.5.0
+yarn add react@^15.5.0 react-dom@^15.5.0
 ```
 
 To install React with npm, run:
 
 ```bash
-npm install --save react@15.5.0 react-dom@15.5.0
+npm install --save react@^15.5.0 react-dom@^15.5.0
 ```
 
 We recommend using a bundler like [webpack](https://webpack.js.org/) or [Browserify](http://browserify.org/) so you can write modular code and bundle it together into small packages to optimize load time.