Replace nginx restart with nginx stop, nginx start.

Fix "start: Job is already running: nginx" error. Abbreviated Output:
+sudo /usr/share/hhvm/install_fastcgi.sh
...
Restarting Nginx
Finished restarting Nginx
+sudo /etc/init.d/hhvm restart
 * Restarting HHVM FastCGI Daemon hhvm       +sudo service nginx restart
nginx start/running, process 5311
start: Job is already running: nginx

tests/before_script.sh

@@ -1,3 +1,4 @@
+set -x
 echo "TRAVIS_PHP_VERSION: ${TRAVIS_PHP_VERSION}"
 
 composer self-update
@@ -54,7 +55,10 @@ server {
 EOF
     sudo /usr/share/hhvm/install_fastcgi.sh
     sudo /etc/init.d/hhvm restart
-    sudo service nginx restart
+    sleep 5
+    sudo service nginx stop
+    sleep 5
+    sudo service nginx start
 elif [[ "${TRAVIS_PHP_VERSION}" == "nightly" ]]; then
     php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
 fi