Update Gemfile.lock to fix vulnerability

Author: matz

Gemfile.lock

@@ -65,7 +65,7 @@ GEM
       listen (= 3.0.6)
       mercenary (~> 0.3)
       minima (= 2.0.0)
-      nokogiri (= 1.6.8.1)
+      nokogiri (~> 1.8.1)
       rouge (= 1.11.1)
       terminal-table (~> 1.4)
     github-pages-health-check (1.3.0)
@@ -76,7 +76,7 @@ GEM
       typhoeus (~> 0.7)
     html-pipeline (2.5.0)
       activesupport (>= 2)
-      nokogiri (>= 1.4)
+      nokogiri (~> 1.8.1)
     i18n (0.7.0)
     jekyll (3.3.1)
       addressable (~> 2.4)