minor #9761 Add testing against HHVM at Travis-CI (stloyd)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #9761).

Discussion
----------

Add testing against HHVM at Travis-CI

Commits
-------

a4d1095 Add testing against HHVM at Travis-CI

.travis.yml

@@ -5,12 +5,17 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
+
+matrix:
+    allow_failures:
+        - php: hhvm
 
 services: mongodb
 
 before_script:
     - sudo apt-get install parallel
-    - echo '' > ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini
+    - sh -c 'if [ $(php -r "echo (int) defined("HHVM_VERSION");") -eq 0 ]; then echo "" >> "~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini"; fi;'
     - echo "extension = mongo.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
     - sh -c 'if [ $(php -r "echo PHP_MINOR_VERSION;") -le 4 ]; then echo "extension = apc.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini; fi;'
     - echo "extension = memcached.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini