add php 7.4 to the build test

.travis.yml

@@ -4,6 +4,10 @@ php:
   - 7.1
   - 7.2
   - 7.3
+  - 7.4snapshot
+  matrix:
+    allow_failures:
+        - php: 7.4snapshot
 env:
   global:
     - ES_VERSION=7.4.0 ES_DOWNLOAD_URL=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-${ES_VERSION}-linux-x86_64.tar.gz