[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        # Use the newer stack for HHVM as HHVM does not support Precise anymore since a long time and so Precise has an outdated version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;
         }