Add support for PHP 7.3

.travis.yml

@@ -15,6 +15,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
 
 install:
   - composer update --no-interaction --prefer-dist $DEPENDENCIES

README.md

@@ -34,6 +34,7 @@ The following versions of PHP are supported:
 * PHP 7.0
 * PHP 7.1
 * PHP 7.2
+* PHP 7.3
 
 The `openssl` extension is also required.