Update list of available access levels (#486)

* Update list of available access levels

Update list of availabe access levels:
- Change `OwnerPermission` to plural
- Rename `Master` to `Maintainer`, see https://about.gitlab.com/2018/06/22/gitlab-11-0-released/#master-role-renamed-to-maintainer
  for more information

* Update access levels for backwards compatibility

gitlab.go

@@ -68,6 +68,10 @@ const (
 	GuestPermissions      AccessLevelValue = 10
 	ReporterPermissions   AccessLevelValue = 20
 	DeveloperPermissions  AccessLevelValue = 30
+	MaintainerPermissions AccessLevelValue = 40
+	OwnerPermissions      AccessLevelValue = 50
+
+	// These are deprecated and should be removed in a future version
 	MasterPermissions     AccessLevelValue = 40
 	OwnerPermission       AccessLevelValue = 50
 )