Lock SSHKit version to fix capistrano load errors

In Capistrano 1.9, SSHKit::DSL needs to be explicitly included:
capistrano/sshkit#219

Author: Karl Sandwich

test/multiverse/suites/capistrano/Envfile

@@ -3,6 +3,7 @@ suite_condition("Capistrano 3 not supported in 1.8.7 and flaky on JRuby") do
 end
 
 boilerplate = <<-BOILERPLATE
+  gem 'sshkit', '~> 1.8.1'
   gem 'rack'
   gem 'net-ssh', '2.9.2' if RUBY_VERSION <= '1.9.3'
   gem 'i18n', '< 0.7.0' if RUBY_VERSION < '1.9.3'