Fix TFClient by removing, rather than adding, a leading '/' character in TFClient.processFeedback()

Fixes issue #78

This is in accordance with both the [hydro migration guidelines](http://wiki.ros.org/hydro/Migration#tf2.2BAC8-Migration.Removal_of_support_for_tf_prefix) and the implementation of TFClient.subscribe() (TFClient.js:102)