Adds homepage and required_ruby_version to gemspec

[davidwessman]

• The homepage adds a link to Rubygems and makes Dependabot
  work better by including the changelog or commit log in PRs.

[adamstegman]

Thanks for filling those out!

[hammady]

@adamstegman Is there a reason for requiring ruby >= 2.6? I am still on 2.3 and it is working perfectly. I know I am behind, but this may hold many people from using later versions of the gem, not even 2.4 or 2.5!

[adamstegman]

@hammady yes, see #213! We dropped support for Ruby versions past their end-of-life. Using Ruby versions older than 2.6 carries a security risk, just like using an old version of ruby-saml that we don't support. For me, it also means fewer possibilities for flaky test as we carry a huge matrix of test versions.

I'd highly encourage you to upgrade your Ruby version, if only for security patches. I don't think there are many breaking changes between 2.3 and 2.6.

[hammady]

@adamstegman Yes, you are right, we are planning the upgrade very soon. I am glad to know there are not much changes required, thanks!