Update nokogiri

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

@@ -333,7 +333,7 @@ GEM
       mime-types-data (~> 3.2015)
     mime-types-data (3.2016.0521)
     mimemagic (0.3.2)
-    mini_portile2 (2.2.0)
+    mini_portile2 (2.3.0)
     minitest (5.10.3)
     multi_json (1.12.1)
     multi_test (0.1.2)
@@ -347,8 +347,8 @@ GEM
       net-ssh (>= 2.6.5)
     net-ssh (4.1.0)
     netrc (0.11.0)
-    nokogiri (1.8.0)
-      mini_portile2 (~> 2.2.0)
+    nokogiri (1.8.1)
+      mini_portile2 (~> 2.3.0)
     notiffany (0.1.1)
       nenv (~> 0.1)
       shellany (~> 0.0)