Revert "add 7.4 to travis CI tests"

This reverts commit 81194f6bd6472f99eeb096e373deaf9f0c5ae4c9.

Found that the trusty based php 7.4 in TravisCI doesn't include the GD module, that we need, plus there is an issue with the Filter::slowEquals() function that only remains for php 5.5 support that doesn't yet contain hash_compare().

.travis.yml

@@ -9,7 +9,6 @@ php:
   - '7.1'
   - '7.2'
   - '7.3'
-  - '7.4'
 
 # as this is a php project, node.js (for JS unit testing) isn't installed
 install: