HHVM: f_xdebug_get_headers is not implemented yet.

HHVM builds should start working as soon as the function is implemented.

The error message was previously: 
> Fatal error: Call to undefined function xdebug_get_headers() in /home/travis/build/mebjas/CSRF-Protector-PHP/test/csrfprotector_test.php on line 28

Now:
> Fatal error: f_xdebug_get_headers is not implemented yet. in /home/travis/build/mebjas/CSRF-Protector-PHP/test/csrfprotector_test.php on line 28

Author: oittaa

.travis.yml

@@ -12,6 +12,7 @@ php:
 matrix:
     allow_failures:
     - php: nightly
+    - php: hhvm
 
 os:
   - linux