Update .travis.yml

Author: gavinjackson

.travis.yml

@@ -1,8 +1,8 @@
 language: php
-php:
-  - 5.6
+php:  
   - 7.2
   - 7.3
-install: composer install
+  - 7.4  
+install: composer update
 script: phpunit test/*.php
 dist: trusty