use latest version of the ssh-slaves plugin

Both release channels for Jenkins ship with v1.5 of the ssh-slaves plugin.  The latest version of ssh-slaves is v1.6, which includes a new optional launch timeout setting.  

Shipping with the lastest version of the ssh-slaves plugin would simplify the groovy script needed to support this new launch timeout setting in the Chef cookbook for Jenkins.  See [opscode/jenkins PR #211](https://github.com/opscode-cookbooks/jenkins/pull/211/files#diff-07b0a2b5973d0ffff6d39365d945fce8R144) for the groovy script that is causing heartburn in the Chef community.