minor symfony#15510 [travis] merge php-nightly and deps=high test-matrix lines (nicolas-grekas)

This PR was merged into the 2.3 branch.

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

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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b4b930d [travis] merge php: nightly and deps=high test-matrix lines

Author: fabpot

.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