minor symfony#14115 let Travis builds fail when PHP 7 jobs fail (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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let Travis builds fail when PHP 7 jobs fail

| 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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ecc3df5 let Travis builds fail when PHP 7 jobs fail

Author: fabpot

.travis.yml

@@ -19,8 +19,6 @@ matrix:
         - php: 5.6
           env: deps=high
         - php: nightly
-    allow_failures:
-        - php: nightly
     fast_finish: true
 
 services: mongodb