SSHKit::Command#environment_string doesn't exist in sshkit 1.3 due to a
bug/typo, causing NoMethodError when running `cap <stage> rails:console`
Bug was fixed in https://github.com/capistrano/sshkit/commit/f3955d1a,
which was included in 1.4.0, so update gemspec to require sshkit >= 1.4