minor symfony#19044 [travis] HHVM 3.12 LTS (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[travis] HHVM 3.12 LTS

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | I guess yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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6da9959 [travis] HHVM 3.12 LTS

Author: fabpot

.travis.yml

@@ -18,7 +18,7 @@ env:
 matrix:
     include:
         # Use the newer stack for HHVM as HHVM does not support Precise anymore since a long time and so Precise has an outdated version
-        - php: hhvm
+        - php: hhvm-3.12
           sudo: required
           dist: trusty
           group: edge