Add project metadata to the gemspec
Add project metadata to the gemspec

As per https://guides.rubygems.org/specification-reference/#metadata,
add metadata to the gemspec file. This'll allow people to more easily
access the source code, raise issues and read the changelog. These
`bug_tracker_uri`, `changelog_uri`, `documentation_uri`, and
`source_code_uri` links will appear on the rubygems page at
https://rubygems.org/gems/rails-html-sanitizer and be available via the
rubygems API after the next release.

rails/rails-html-sanitizer