bug #21923 [travis] Test with hhvm 3.18 (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
----------

[travis] Test with hhvm 3.18

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #21900
| License       | MIT
| Doc PR        | -

Needs #21922 on master to be green also.
Works around  facebook/hhvm#7722.

Commits
-------

7f1f0cb630 [travis] Test with hhvm 3.18

.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        # Use the newer stack for HHVM as HHVM does not support Precise anymore since a long time and so Precise has an outdated version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-3.15
+        - php: hhvm-3.18
           sudo: required
           dist: trusty
           group: edge

src/Symfony/Bundle/SecurityBundle/Tests/Functional/WebTestCase.php

@@ -25,7 +25,7 @@ class WebTestCase extends BaseWebTestCase
 
     protected static function deleteTmpDir($testCase)
     {
-        if (!file_exists($dir = sys_get_temp_dir().'/'.Kernel::VERSION.'/'.$testCase)) {
+        if (defined('HHVM_VERSION_ID') || !file_exists($dir = sys_get_temp_dir().'/'.Kernel::VERSION.'/'.$testCase)) {
             return;
         }