remove use of hacks

This hack:

https://github.com/osrf/capabilities/blob/master/scripts/capability_server#L5-L22

Can be removed since `rospy` version `1.9.50` is now out:

http://www.ros.org/debbuild/hydro.html?q=rospy
http://www.ros.org/debbuild/groovy.html?q=rospy

We should also investigate making an upstream fix/change for this hack:

https://github.com/osrf/capabilities/blob/master/src/capabilities/server.py#L91-L116