Only re-write docs _config on version bumps

Doesn't fix, but mostly addresses the concerns in #24. Some churn at
version bumps is far better than what we have right now.

Rakefile

@@ -23,8 +23,10 @@ desc "update version to match ../package.json"
 task :update_version do
   react_version = JSON.parse(File.read('../package.json'))['version']
   site_config = YAML.load_file('_config.yml')
+  if site_config['react_version'] != react_version
     site_config['react_version'] = react_version
     File.open('_config.yml', 'w+') { |f| f.write(site_config.to_yaml) }
+  end
 end
 
 desc "build into ../../react-gh-pages"