travis: added PHP 7.3

Author: dg

.travis.yml

@@ -2,6 +2,7 @@ language: php
 php:
     - 7.1
     - 7.2
+    - 7.3
 
 before_install:
     # turn off XDebug

readme.md

@@ -38,4 +38,4 @@ The recommended way to install is via Composer:
 composer require nette/utils
 ```
 
-It requires PHP version 5.6 and supports PHP up to 7.2. The dev-master version requires PHP 7.1.
+It requires PHP version 5.6 and supports PHP up to 7.3. The dev-master version requires PHP 7.1.