We have to specify requirements for librarian

Otherwise librarian-puppet fails to read version. It may vary on
different rubygems version which are used for version parsing however on
ruby 1.8.7 with rubygems 1.8 this is required.
Drop 2.6 support
Bump stdlib min to 2.0 to fix Forge upload error
Release 1.4.0
Fix top-scope variable without an explicit namespace issue
Fixes #3850 - Accept an array of reverse DNS zones