install proxy-related headers only if zookeeper is enabled (fix #517)

Excluded proxy.hp, proxy_common.hpp and aggregators.hpp when --enable-zookeeper is not specified. (fix for #517)

My original goal of the issue #517 is to avoid confusing error message when using jubatus-service-skeleton against Jubatus compiled without ZooKeeper support, and the goal is achieved by this change:

```
../kvs_proxy.cpp:13:46: fatal error: jubatus/server/framework/proxy.hpp: No such file or directory
```