upgrade elasticsearch version in the travis to the 7.5.2

Author: saimaz

.travis.yml

@@ -8,7 +8,7 @@ php:
   - 7.4
 env:
   global:
-    - ES_VERSION=7.4.0 ES_DOWNLOAD_URL=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-${ES_VERSION}-linux-x86_64.tar.gz
+    - ES_VERSION=7.5.2 ES_DOWNLOAD_URL=https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-${ES_VERSION}-linux-x86_64.tar.gz
 install:
   - wget ${ES_DOWNLOAD_URL}
   - tar -xzf elasticsearch-${ES_VERSION}-linux-x86_64.tar.gz