[travis] Add PHP 7 to allowed failures until segfaults are fixed

Author: nicolas-grekas

.travis.yml

@@ -14,9 +14,12 @@ matrix:
         - php: 5.4
         - php: 5.5
         - php: 5.6
-          env: deps=low
-        - php: 7.0
           env: deps=high
+        - php: 7
+        - php: 5.6
+          env: deps=low
+    allow_failures:
+        - php: 7
     fast_finish: true
 
 services: mongodb