[travis] merge php: nightly and deps=high test-matrix lines

Author: nicolas-grekas

.travis.yml

@@ -16,9 +16,8 @@ matrix:
         - php: 5.5
         - php: 5.6
           env: deps=low
-        - php: 5.6
-          env: deps=high
         - php: 7.0
+          env: deps=high
     fast_finish: true
 
 services: mongodb