Change "OS X" references to "macOS" (facebook#1511)

Updated README.md to refer to the current rebranding.

template/README.md

@@ -578,7 +578,7 @@ set REACT_APP_SECRET_CODE=abcdef&&npm start
 
 (Note: the lack of whitespace is intentional.)
 
-#### Linux, OS X (Bash)
+#### Linux, macOS (Bash)
 
 ```bash
 REACT_APP_SECRET_CODE=abcdef npm start
@@ -684,7 +684,7 @@ set HTTPS=true&&npm start
 
 (Note: the lack of whitespace is intentional.)
 
-#### Linux, OS X (Bash)
+#### Linux, macOS (Bash)
 
 ```bash
 HTTPS=true npm start
@@ -912,7 +912,7 @@ set CI=true&&npm run build
 
 (Note: the lack of whitespace is intentional.)
 
-##### Linux, OS X (Bash)
+##### Linux, macOS (Bash)
 
 ```bash
 CI=true npm test
@@ -1285,7 +1285,7 @@ If this doesn’t happen, try one of the following workarounds:
 * If the watcher doesn’t see a file called `index.js` and you’re referencing it by the folder name, you [need to restart the watcher](https://github.com/facebookincubator/create-react-app/issues/1164) due to a Webpack bug.
 * Some editors like Vim and IntelliJ have a “safe write” feature that currently breaks the watcher. You will need to disable it. Follow the instructions in [“Working with editors supporting safe write”](https://webpack.github.io/docs/webpack-dev-server.html#working-with-editors-ides-supporting-safe-write).
 * If your project path contains parentheses, try moving the project to a path without them. This is caused by a [Webpack watcher bug](https://github.com/webpack/watchpack/issues/42).
-* On Linux and OS X, you might need to [tweak system settings](https://webpack.github.io/docs/troubleshooting.html#not-enough-watchers) to allow more watchers.
+* On Linux and macOS, you might need to [tweak system settings](https://webpack.github.io/docs/troubleshooting.html#not-enough-watchers) to allow more watchers.
 
 If none of these solutions help please leave a comment [in this thread](https://github.com/facebookincubator/create-react-app/issues/659).