[MINOR] Addressed Docs dependency security vulnerability

Addressed issue raised on mailing list:
https://www.mail-archive.com/devlivy.incubator.apache.org/msg00295.html

More info on the vulnerability:
https://snyk.io/blog/nokogiri-xxe-vulnerabilities/

Author: Alex Bozarth <ajbozart@us.ibm.com>

Closes apache#75 from ajbozarth/fix.

docs/Gemfile.lock

@@ -71,7 +71,7 @@ GEM
       typhoeus (~> 0.7)
     html-pipeline (2.6.0)
       activesupport (>= 2)
-      nokogiri (>= 1.4)
+      nokogiri (>= 1.5.4)
     i18n (0.8.6)
     jekyll (3.4.5)
       addressable (~> 2.4)