Make zuul proxy route filter retriable

Hello,

After make any tests on RC1 I have discovered that the Zuul proxy route does not implement the Ribbon retry.
If any instance registered in Eureka do not respond the proxy fail.

This commit activate the retry handler on for the RibbonCommand on ZuulProxy.

Do you think that is it necessary to make this feature configurable ( For example in a  zuul.routes.XXX.retriable properties ) ?
I can update my PR with this modification if you want.

Thanks a lot.
Julien.