Update nokogiri to 1.8.2

In response to security advisory reported by bundler audit:

```
Name: nokogiri
Version: 1.8.1
Advisory: CVE-2017-15412
Criticality: Unknown
URL: sparklemotion/nokogiri#1714
Title: Nokogiri gem, via libxml, is affected by DoS vulnerabilities
Solution: upgrade to >= 1.8.2

Vulnerabilities found!
```

Gemfile.lock

@@ -58,7 +58,7 @@ GEM
     minitest (5.10.3)
     multipart-post (2.0.0)
     netrc (0.11.0)
-    nokogiri (1.8.1)
+    nokogiri (1.8.2)
       mini_portile2 (~> 2.3.0)
     octokit (4.7.0)
       sawyer (~> 0.8.0, >= 0.5.3)