Fix support to Ruby 2.1

Author: rafaelfranca

lib/sdoc/generator.rb

@@ -193,9 +193,10 @@ def generate_class_tree_level(classes, visited = {})
 
   ### Determines index path based on @options.main_page (or lack thereof)
   def index_path
+    main_page = @options.main_page
     # Transform class name to file path
-    if @options.main_page&.include?("::")
-      slashed = @options.main_page.sub(/^::/, "").gsub("::", "/")
+    if main_page && main_page.include?("::")
+      slashed = main_page.sub(/^::/, "").gsub("::", "/")
       "%s/%s.html" % [ class_dir, slashed ]
     else
       index.path