add php7.4 to the build matrix

Author: wissem

.travis.yml

@@ -5,6 +5,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.4
 
 before_install:
   - if [ -n "$GH_TOKEN" ]; then composer config github-oauth.github.com ${GH_TOKEN}; fi;