Added migration notification for rails plugin

plugins/rails3/rails3.plugin.zsh

+echo "It looks like you have been using the 'rails3' plugin,"
+echo "which has been deprecated in favor of a newly consolidated 'rails' plugin."
+echo "You will want to modify your ~/.zshrc configuration to begin using it."
+echo "Learn more at https://github.com/robbyrussell/oh-my-zsh/pull/2240"

plugins/rails4/rails4.plugin.zsh

+echo "It looks like you have been using the 'rails4' plugin,"
+echo "which has been deprecated in favor of a newly consolidated 'rails' plugin."
+echo "You will want to modify your ~/.zshrc configuration to begin using it."
+echo "Learn more at https://github.com/robbyrussell/oh-my-zsh/pull/2240"