[TEST] build against ES 6 on Travis

.travis.yml

@@ -14,24 +14,28 @@ branches:
 
 sudo: true
 
-php:
-  - 7.0
-  - 7.1
-
 matrix:
   fast_finish: true
   include:
+    - php: 7.0
+      env: ES_VERSION="5.5" TEST_BUILD_REF="origin/5.5"
+    - php: 7.0
+      env: ES_VERSION="6.0" TEST_BUILD_REF="origin/6.0"
+    - php: 7.0
+      env: ES_VERSION="6.x" TEST_BUILD_REF="origin/6.x"
+
     - php: 7.1
-      env: ES_VERSION="5.0" TEST_BUILD_REF="origin/5.0"
+      env: ES_VERSION="5.5" TEST_BUILD_REF="origin/5.5"
     - php: 7.1
-      env: ES_VERSION="5.1" TEST_BUILD_REF="origin/5.1"
+      env: ES_VERSION="6.0" TEST_BUILD_REF="origin/6.0"
     - php: 7.1
-      env: ES_VERSION="5.2" TEST_BUILD_REF="origin/5.2"
+      env: ES_VERSION="6.x" TEST_BUILD_REF="origin/6.x"
+
+  allow_failures:
+    - env: ES_VERSION="6.x" TEST_BUILD_REF="origin/6.x"
 
 env:
   global:
-    - ES_VERSION="5.x"
-    - TEST_BUILD_REF="origin/5.x"
     - ES_TEST_HOST=http://localhost:9200
     - JAVA_HOME="/usr/lib/jvm/java-8-oracle/jre"