When GHOrganization#publicize method argument is a GHUser that is
already publicizing its membership, GitHub returns error 500, causing
the API to fail.

This commit fixes this behaviour with a prior verification whether
the organization has the user or not.

This verification is made with GHOrganization#hasPublicMembers(GHUser).
As this method is not available in github-api-1.28, the pom.xml was
updated to version 1.37, which contains the missing method.