update travis ci config for PHP 7.2, 7.3

Author: JeremyDunn

.travis.yml

@@ -1,12 +1,13 @@
 language: php
 
 php:
-  - 5.6
   - 7.0
   - 7.1
+  - 7.2
+  - 7.3
 
 install:
-  - composer update --no-interaction --no-ansi --no-progress --no-suggest --optimize-autoloader --prefer-stable --prefer-lowest
+  - composer update --no-interaction --no-ansi --no-progress --no-suggest --optimize-autoloader --prefer-stable
 
 script: vendor/bin/phpunit