Fix docs generation for 1.2.0

Author: eregon

support/generate_docs.rb

@@ -21,14 +21,15 @@ def sh(*args)
   puts
   puts version
   sh "git", "checkout", "v#{version}"
+  has_docs = Dir.exist?('docs')
 
   sh "rm", "-f", "Gemfile.lock"
   sh "bundle", "install"
   sh "bundle", "exec", "rake", "yard:#{version}"
 
   sh "cp", "-R", "docs/#{version}", "site"
   sh "rm", "-rf", "docs/#{version}"
-  sh "git", "restore", "docs"
+  sh "git", "restore", "docs" if has_docs
   sh "git", "restore", "docs-source"
 end