let @version be always externally set in guides generation [fixes #8221]

Shelling out was there for authors convenience, but we are
rather going to have the tag or SHA1 always in RAILS_VERSION
and if the environment variable is blank, then just use
"local" as a reminder that you are just working locally.

The docs server has been updated to set the long SHA1 in
RAILS_VERSION when generating edge guides.

guides/rails_guides/generator.rb

@@ -84,7 +84,7 @@ module RailsGuides
       @warnings = ENV['WARNINGS'] == '1'
       @all      = ENV['ALL']      == '1'
       @kindle   = ENV['KINDLE']   == '1'
-      @version  = ENV['RAILS_VERSION'] || `git rev-parse --short HEAD`.chomp
+      @version  = ENV['RAILS_VERSION'] || 'local'
       @lang     = ENV['GUIDES_LANGUAGE']
     end
 

guides/source/_welcome.html.erb

@@ -1,4 +1,4 @@
-<h2>Ruby on Rails Guides (<%= @version %>)</h2>
+<h2>Ruby on Rails Guides (<%= @edge ? @version[0, 7] : @version %>)</h2>
 
 <% if @edge %>
 <p>