Update nokogiri

The currently installed version has a security advisory:

```
Updated ruby-advisory-db
ruby-advisory-db: 287 advisories
Name: nokogiri
Version: 1.7.1
Advisory: CVE-2017-5029
Criticality: Unknown
URL: sparklemotion/nokogiri#1634
Title: Nokogiri gem contains two upstream vulnerabilities in libxslt 1.1.29
Solution: upgrade to >= 1.7.2
```

Gemfile.lock

@@ -47,11 +47,11 @@ GEM
     iso_country_codes (0.7.1)
     json (1.8.2)
     method_source (0.8.2)
-    mini_portile2 (2.1.0)
+    mini_portile2 (2.2.0)
     minitest (5.9.0)
     multipart-post (2.0.0)
-    nokogiri (1.7.1)
-      mini_portile2 (~> 2.1.0)
+    nokogiri (1.8.0)
+      mini_portile2 (~> 2.2.0)
     octokit (3.8.0)
       sawyer (~> 0.6.0, >= 0.5.3)
     parser (2.3.3.1)