Update nokogiri 1.7.2 --> 1.8.1

Security vulnerabilities identified in libxml2
as used by nokogiri.

Affected versions: Prior to 1.8.1
Fixed versions: 1.8.1
Identifier: USN-3424-1
Solution: Upgrade to latest version.
Source: sparklemotion/nokogiri#1673

Gemfile.lock

@@ -383,7 +383,7 @@ GEM
       mime-types-data (~> 3.2015)
     mime-types-data (3.2016.0521)
     mini_magick (4.2.0)
-    mini_portile2 (2.1.0)
+    mini_portile2 (2.3.0)
     minitest (5.10.1)
     multi_json (1.12.1)
     multi_xml (0.5.5)
@@ -393,8 +393,8 @@ GEM
       net-ssh (>= 2.6.5)
     net-ssh (2.9.2)
     netaddr (1.5.0)
-    nokogiri (1.7.2)
-      mini_portile2 (~> 2.1.0)
+    nokogiri (1.8.1)
+      mini_portile2 (~> 2.3.0)
     notiffany (0.0.8)
       nenv (~> 0.1)
       shellany (~> 0.0)