Fix rspec warning

Author: biow0lf

spec/errbit_github_plugin/issue_tracker_spec.rb

@@ -152,7 +152,7 @@
         expect(Octokit::Client).to receive(:new).with(
           login: user["github_login"], access_token: user["github_oauth_token"]
         ).and_raise(Octokit::Unauthorized)
-        expect { subject }.to raise_error
+        expect { subject }.to raise_error(ErrbitGithubPlugin::AuthenticationError)
       end
     end
   end