Add support for php 7.4

Author: snapshotpl

.travis.yml

@@ -24,6 +24,10 @@ matrix:
       env: ES_VERSION="7.4.2"
     - php: 7.3
       env: ES_VERSION="8.0.0-SNAPSHOT"
+    - php: 7.4
+      env: ES_VERSION="7.4.2"
+    - php: 7.4
+      env: ES_VERSION="8.0.0-SNAPSHOT"
   allow_failures:
     - php: 7.3
       env: ES_VERSION="8.0.0-SNAPSHOT"