Change Since from int to int64 in OrganizationsListOptions. (google#899)

Field Since in OrganizationsListOptions refers to an organization ID,
so its type needs to be int64 to per prior decision in google#597.
This is already the case in other similar structs such as
RepositoryListAllOptions and UserListOptions.

This is a breaking API change that is a part of resolving issue google#597.
It should've been applied earlier, but we missed it (likely because
the field doesn't have "ID" in its name, so it's harder to spot).

Updates google#597.

Author: qinfchen

github/orgs.go

@@ -75,7 +75,7 @@ func (p Plan) String() string {
 // OrganizationsService.ListAll method.
 type OrganizationsListOptions struct {
 	// Since filters Organizations by ID.
-	Since int `url:"since,omitempty"`
+	Since int64 `url:"since,omitempty"`
 
 	ListOptions
 }

github/orgs_test.go

@@ -18,7 +18,7 @@ func TestOrganizationsService_ListAll(t *testing.T) {
 	client, mux, _, teardown := setup()
 	defer teardown()
 
-	since := 1342004
+	since := int64(1342004)
 	mux.HandleFunc("/organizations", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "GET")
 		testHeader(t, r, "Accept", mediaTypeGraphQLNodeIDPreview)