Convert !~ call into !include?

This particular case is safe to convert, since super should always
return a string.

Author: jeremyevans

lib/roda/plugins/view_options.rb

@@ -182,7 +182,7 @@ def render_layout_opts
         # contain a slash.
         def template_name(opts)
           name = super
-          if (v = @_view_subdir) && name !~ /\//
+          if (v = @_view_subdir) && !name.include?('/')
             "#{v}/#{name}"
           else
             name