Bump octokit version to work around Faraday issue

Without this, we get failing builds with:

```
NameError: uninitialized constant Faraday::Error::ClientError
Did you mean?  Faraday::ClientError
```

This is documented in octokit/octokit.rb#1154.

Gemfile.lock

@@ -34,7 +34,8 @@ GEM
     hashdiff (0.3.7)
     method_source (0.9.2)
     multipart-post (2.1.1)
-    octokit (4.13.0)
+    octokit (4.22.0)
+      faraday (>= 0.9)
       sawyer (~> 0.8.0, >= 0.5.3)
     pry (0.12.2)
       coderay (~> 1.1.0)