Merge pull request #241 from MarTango/martango/update-mac-installation-instructions

Update Mac OS PHP installation instructions

docs/GettingStarted.md

@@ -18,8 +18,7 @@ If it's not PHP 7.1, then you have a few different options for how to access it:
 1. Install [Homebrew](http://brew.sh/)
 2. Install PHP 7.1 and Composer
 ```
-brew tap homebrew/php
+brew install php@7.1
-brew install php71
 brew install composer
 ```