drop php 5.5 tests, allowing us to use current ubuntu bionic based environment and adding php 7.4 incl. GD library

.travis.yml

@@ -1,14 +1,12 @@
 language: php
 sudo: false
-# only needed for PHP 5.5 support as of 2019-07
-dist: trusty
 php:
-  - '5.5'
   - '5.6'
   - '7.0'
   - '7.1'
   - '7.2'
   - '7.3'
+  - '7.4'
 
 # as this is a php project, node.js (for JS unit testing) isn't installed
 install: