[SourcesManager] Inform users about pre-release updates

Closes https://github.com/CocoaPods/CocoaPods/issues/2460.

lib/cocoapods/sources_manager.rb

@@ -199,8 +199,8 @@ module Pod
       #
       def check_version_information(dir)
         versions = version_information(dir)
-        unless repo_compatible?(dir)
         min, max = versions['min'], versions['max']
+        unless repo_compatible?(dir)
           version_msg = ( min == max) ? min : "#{min} - #{max}"
           raise Informative, "The `#{dir.basename}` repo requires " \
           "CocoaPods #{version_msg} (currently using #{Pod::VERSION})\n".red +
@@ -210,9 +210,11 @@ module Pod
         needs_sudo = path_writable?(__FILE__)
 
         if config.new_version_message? && cocoapods_update?(versions)
+          install_message = needs_sudo ? 'sudo ' : ''
+          install_message << 'gem install cocoapods'
+          install_message << ' --pre' if Gem::Version.new(max).prerelease?
           UI.puts "\nCocoaPods #{versions['last']} is available.\n" \
-            "To update use: #{needs_sudo ? 'sudo ' : ''}" \
-            'gem install cocoapods\n'.green
+            "To update use: `#{install_message}`\n".green
         end
       end