Fix Travis build

As of commit f9d1d1f[1] we require `sudo`, so we tell Travis about it.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=f9d1d1f>

.travis.yml

@@ -1,6 +1,6 @@
 dist: trusty
 language: c
-sudo: false
+sudo: required
 addons:
   apt:
     packages: