Handle Fixnum and Bignum being merged into Integer in Ruby 2.4

[exastencil]

I've noticed that Paggio doesn't load in versions of Ruby since 2.4. When monkey-patching numeric classes to handle CSS unit calculations it raises when it encounters Fixnum which no longer exists since Ruby 2.4.0.

I'm not sure whether the Travis CI or Github workflows are being honoured because Travis CI doesn't have versions after 2.4 so this is the only place specs should pass, but the latest commit references the Github workflow and all versions there are after 2.4 so it seems the goal is not to support pre-2.4 Rubies anymore. So I've added the major versions to both.

In case the goal is to support older and newer Ruby versions I put a conditional to only patch the correct classes. My recommendation is to release a new version, and set the gemspec to require at least Ruby 2.4 and do away with testing the older versions. I didn't want to presume and do it that, but let me know and I can make that change.