[Github] Add installation_repositories event (#55)

I've recently been using your type definitions while playing around with
the Github Webhooks and I realised the `install_repositories` event was
missing. I've added it and updated the tests.

github/github_test.go

@@ -213,6 +213,16 @@ func TestWebhooks(t *testing.T) {
 				"X-Hub-Signature": []string{"sha1=2058cf6cc28570710afbc638e669f5c67305a2db"},
 			},
 		},
+		{
+			name:     "InstallationRepositoriesEvent",
+			event:    InstallationRepositoriesEvent,
+			typ:      InstallationRepositoriesPayload{},
+			filename: "../testdata/github/installation-repositories.json",
+			headers: http.Header{
+				"X-Github-Event":  []string{"installation_repositories"},
+				"X-Hub-Signature": []string{"sha1=c587fbd9dd169db8ae592b3bcc80b08e2e6f4f45"},
+			},
+		},
 		{
 			name:     "IntegrationInstallationEvent",
 			event:    IntegrationInstallationEvent,