fix error on alias listing

see
http://danbooru.donmai.us/forum_topics/9127?page=85#forum_post_105019

app/models/tag_alias.rb

@@ -112,11 +112,11 @@ class TagAlias < ActiveRecord::Base
   end
 
   def antecedent_tag
-    Tag.find_by_name(antecedent_name)
+    Tag.find_or_create_by_name(antecedent_name)
   end
 
   def consequent_tag
-    Tag.find_by_name(consequent_name)
+    Tag.find_or_create_by_name(consequent_name)
   end
 
   def absence_of_transitive_relation

app/models/tag_implication.rb

@@ -192,11 +192,11 @@ class TagImplication < ActiveRecord::Base
   end
 
   def antecedent_tag
-    Tag.find_by_name(antecedent_name)
+    Tag.find_or_create_by_name(antecedent_name)
   end
 
   def consequent_tag
-    Tag.find_by_name(consequent_name)
+    Tag.find_or_create_by_name(consequent_name)
   end
 
   def reload(options = {})