Update nokogiri to version 1.6.8.1

Author: sferik

Diff for: Gemfile.lock

@@ -97,14 +97,12 @@ GEM
     mini_portile2 (2.1.0)
     minitest (5.9.1)
     nested_form (0.3.2)
-    nokogiri (1.6.8)
+    nokogiri (1.6.8.1)
       mini_portile2 (~> 2.1.0)
-      pkg-config (~> 1.1.7)
     orm_adapter (0.5.0)
     parser (2.3.1.4)
       ast (~> 2.2)
     pg (0.19.0)
-    pkg-config (1.1.7)
     powerpack (0.1.1)
     puma (3.6.0)
     rack (1.6.4)