acme_updater: call apache2ctl by it's absolute path (fixes #6)

acme_updater.py

@@ -183,7 +183,7 @@ for entry in PARSED_VHOSTS:
 if CERT_RENEWED:
     LOGGER.debug("Checking apache configuration")
     try:
-        subprocess.check_call(["apache2ctl", "-t"])
+        subprocess.check_call(["/usr/sbin/apache2ctl", "-t"])
     except subprocess.CalledProcessError:
         LOGGER.error("Error in apache configuration, will not restart the "
                      "web server")