Update provision.sh

The *xdebug_on* command no longer works, as *php5enmod* tries to create symlinks in both the */etc/php5/fpm/conf.d* and */etc/php5/cli/conf.d* directories that point to */etc/php5/mods-available/xdebug.ini*. This fails with the following message:
`WARNING: Module xdebug ini file doesn't exist under /etc/php5/mods-available`

As it stands, xdebug.ini is placed in */etc/php5/fpm/conf.d* and loaded all the time  but xdebug is not active because of the absence of a valid `zend_extension` path. 

This fix updates provision.sh to place xdebug.ini in the mods-available directory so that `xdebug_on` can create the needed symlinks in both `/etc/php5/fpm/conf.d` and`/etc/php5/cli/conf.d` CLI. It also finds the path to xdebug.so and prepends it to xdebug.ini so the the extension can be loaded.

Of note - opcache.ini remains in the fpm/conf.d directory and is always loaded in FPM, alongside mods-available/opcache.ini - this may require similar attention to xdebug.ini