Add new Ruby releases and bump the default to 2.6.6
Add new Ruby releases and bump the default to 2.6.6

These fix several bugs but primarily they were all released to fix these CVEs:

    CVE-2020-16255: Unsafe Object Creation Vulnerability in JSON (Additional fix)
    CVE-2020-10933: Heap exposure vulnerability in the socket library

Signed-off-by: Tim Smith <764ef62106582a09ed09dfa0b6bff7c05fd7d1e4@chef.io>

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