Bump repomd_parser to 0.1.6 for ztd and pin gems which have 2.6+ dependency on later versions

[felixsch]

part of: https://trello.com/c/BfrlfQwR/3154-support-zst-compression-format-in-rmt
Also handles: #1045 and #1042
fixes: #1050

This is updating repomd_parser which now comes with zstd support and pinning gems which updates now requires ruby 2.6+ to work.

How to review this pull request:

• Run bundle and see updates coming through

• Check zstd support:

$ bin/rmt-cli repos custom add http://download.opensuse.org/tumbleweed/repo/oss/ tumble-oss
$ bin/rmt-cli mirror repository tumble-oss
# expect: Wait until it starts downloading `rpm` packages to know it successfully parsed the `primary.xml.zst` file

Thank you for reviewing this pull request! If you have questions, please do not hesitate to ask! 🚀

[ngetahun]

Support zst compression format in RMT

[ngetahun]

LGTM! Tested with the given demo instructions