ProxyPassReverse should refer to balancer instead of back ends

When Apache is used for load balancing, directive `ProxyPassReverse` should refer to the url of a balancer instead of the individual urls of the back end systems.

Currently, it directly refers to the two Play instances:

    ProxyPassReverse / http://localhost:9999/
    ProxyPassReverse / http://localhost:9998/

However, I think it should refer to the balancer instead:

    ProxyPassReverse / balancer://mycluster/

See also: [Examples of a balancer configuration](http://httpd.apache.org/docs/current/mod/mod_proxy_balancer.html#example) in the Apache Module mod_proxy_balancer documentation.