Add the vagrant user to the www-data group for xdebug log access

When running php (or phpunit) inside the VM rather than the web
interface, an xdebug-remote.log access error would appear in some
scenarios. In these cases, php runs as the vagrant user rather than
the www-data user and does not have access to the log files.

This modifies the vagrant user during provisioning so that one of
its alternate groups is www-data.

See #691, #631.
Fixes #621.

provision/provision.sh

@@ -410,6 +410,10 @@ php5enmod mailcatcher
 
 service php5-fpm restart
 
+# Add the vagrant user to the www-data group so that it has better access
+# to PHP and Nginx related files.
+usermod -a -G www-data vagrant
+
 # If MySQL is installed, go through the various imports and service tasks.
 exists_mysql="$(service mysql status)"
 if [[ "mysql: unrecognized service" != "${exists_mysql}" ]]; then