Removed php 7.4 from travis

MaximilianKresse · MaximilianKresse · commit fd8db773b932 · 2019-11-29T11:28:56.000+01:00
(it isn't supported yet and phpunit 7 doesn't support it)
diff --git a/.travis.yml b/.travis.yml
@@ -4,7 +4,6 @@ php:
   - '7.1'
   - '7.2'
   - '7.3'
-  - '7.4'
 
 before_script:
   - composer selfupdate
