Update all Bintray URLs for the new git-for-windows organization

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
readme: Update to not contain "mingwGitDevEnv" anymore, except mailing list

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
readme: Update the mailing list URL

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>