update capistrano, sshkit dependencies

Updated capistrano dependency to latest version 3.1 and removed sshkit dependency as this is included in the capistrano dependencies (~> 1.3)

At the moment it is not possible to use it with capistrano 3.1.
It is also creating a gem conflict when the latest capistrano-rails (1.1) is used as this depends on capistrano 3.1