Replace custom metadata system with standard gemspec, Travis CI with
GitHub Actions, and add Rakefile. Simplify version handling, remove
Ruby 1.8/1.9 compat code. Inline Hash#rekey to drop facets dependency
from neapolitan itself (malt still needs it). Convert docs to markdown.
Update URLs, clean up obsolete files.

Note: tests require malt to be updated for Ruby 3+ before they will pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>