Backport https://github.com/ruby/shell/pull/1

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66354 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/shell.rb

@@ -17,6 +17,7 @@ require "forwardable"
 require "shell/error"
 require "shell/command-processor"
 require "shell/process-controller"
+require "shell/version"
 
 # Shell implements an idiomatic Ruby interface for common UNIX shell commands.
 #