CI workflow fails on Ruby 1.9.3 due rails-html-sanitizer 1.5.0 release

[v-kolesnikov]

I've noticed a fail on #111 and run workflow agains master branch and got the same issue:
https://github.com/v-kolesnikov/forme/actions/runs/4023984568/jobs/6915486549#step:4:15

Quick glance lead me here: rails/rails-html-sanitizer@v1.4.4...v1.5.0#diff-0daf33b9062eb5ccdeae86ed8bf2662a6e8669f1a7db590802b7f3b36ea47426R113

rails-html-sanitizer was released recently as 1.5.0 that is incompatible with Ruby 1.9.3, but supported Ruby version is not specified in this gem.

Possible solutions:

• Specify rails-html-sanitizer version manually for CI workflow as 1.4.4 for Ruby 1.9.3 by creating a custom Gemfile
• Do not run Rails integration test on Ruby 1.9.3
• Stop support Ruby 1.9.3 for Forme

Ready to help with this.

[jeremyevans]

This type of issue pops up occasionally for older Ruby versions. I generally work around it by specifying gem version constraints based on the Ruby version. I'll take care of this after merging #111.