ruby: bump to 2.4.2

ruby: bump to 2.4.2

This release contains some security fixes.

 CVE-2017-0898: Buffer underrun vulnerability in Kernel.sprintf
 CVE-2017-10784: Escape sequence injection vulnerability in the Basic authentication of WEBrick
 CVE-2017-14033: Buffer underrun vulnerability in OpenSSL ASN1 decode
 CVE-2017-14064: Heap exposure in generating JSON
 Multiple vulnerabilities in RubyGems
 Update bundled libyaml to version 0.1.7.

And many other bugfix.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>