Lock Homestead version because master is unpredictable

Author: lcharette

vagrant/README.md

@@ -12,7 +12,7 @@ UserFrosting uses the [Laravel/Homestead](https://laravel.com/docs/5.6/homestead
 * Clone Homestead from the root of your cloned fork of the UserFrosting Git repository
 
 ```sh
-git clone https://github.com/laravel/homestead.git vagrant/Homestead
+git clone -b v7.20.0 https://github.com/laravel/homestead.git vagrant/Homestead
 ```
 
 * Run `vagrant up` from the root of your cloned fork of the UserFrosting Git repository
@@ -127,4 +127,4 @@ $ mysql -uhomestead -psecret UserFrosting < /home/vagrant/userfrosting/userfrost
 - Username: homestead
 - Password: secret
 - Database: UserFrosting
-- Port: 5432
+- Port: 5432