Change ParentTeamID field to int (from string). (#748)

Using the code as written in the `NestedTeamsPreview` branch produces
an error when talking to the GitHub API:

	panic: PATCH https://github.com/api/teams/2514825: 422 Invalid request.

	For 'properties/parent_team_id', "2514824" is not a number or null. []

The documentation at https://developer.github.com/v3/orgs/teams/#create-team
states that this parameter is an id type which appears to mean an integer, not a string.
Also, id is an integer in the sample responses for Get team endpoint, etc.

Author: alindeman

github/github-accessors.go

@@ -117,7 +117,7 @@ type NewTeam struct {
 	Description  *string  `json:"description,omitempty"`
 	Maintainers  []string `json:"maintainers,omitempty"`
 	RepoNames    []string `json:"repo_names,omitempty"`
-	ParentTeamID *string  `json:"parent_team_id,omitempty"`
+	ParentTeamID *int     `json:"parent_team_id,omitempty"`
 
 	// Deprecated: Permission is deprecated when creating or editing a team in an org
 	// using the new GitHub permission model. It no longer identifies the