Add support for new Rails 5 API, redirect_back (tpope#467)

Author: latortuga

autoload/rails.vim

@@ -3787,7 +3787,7 @@ function! rails#buffer_syntax()
         syn keyword rubyRailsRenderMethod render
         syn keyword rubyRailsMethod logger polymorphic_path polymorphic_url
         syn keyword rubyRailsControllerMethod helper helper_attr helper_method filter layout url_for serialize exempt_from_layout filter_parameter_logging hide_action cache_sweeper protect_from_forgery caches_page cache_page caches_action expire_page expire_action rescue_from
-        syn keyword rubyRailsRenderMethod head redirect_to render_to_string respond_with
+        syn keyword rubyRailsRenderMethod head redirect_to redirect_back render_to_string respond_with
         syn match   rubyRailsRenderMethod '\<respond_to\>?\@!'
         syn keyword rubyRailsFilterMethod before_filter append_before_filter prepend_before_filter after_filter append_after_filter prepend_after_filter around_filter append_around_filter prepend_around_filter skip_before_filter skip_after_filter skip_filter before_action append_before_action prepend_before_action after_action append_after_action prepend_after_action around_action append_around_action prepend_around_action skip_before_action skip_after_action skip_action
         syn keyword rubyRailsFilterMethod verify