Upgrade stdlib from 2.6.2 to 2.6.5

[nirvdrum]

I'm getting started with a new Rails project and the mismatch between TruffleRuby's latest stdlib version and MRI's is a bit annoying. This is because the generated Gemfile from rails new adds a ruby version directive. Since the latest TruffleRuby and MRI have different reported stdlib versions, I can't readily share the Gemfile between the two implementations. The directive is optional, so for now I've just removed it. However, it would be a better out-of-the-box experience Rails experience if I didn't need to do that.

[eregon]

We plan to upgrade to 2.6.4 although I'm not sure yet when it will land. One of the other reasons is to restore rdoc support now that the related vulnerability has been fixed upstream.

Note however that using the same Gemfile for both MRI and TruffleRuby might not always work.
For instance, rails new generates a different Gemfile on MRI on Rails 5 & 6 IIRC (adding MRI-specific gems like for debugging only if rails new ran on MRI).
I think we should fix that in Rails by using Bundler platforms, so it's the same Gemfile generated for both MRI and TruffleRuby.

Practically, using the Gemfile generated by rails new ran on MRI for TruffleRuby should work fine though.

Another thing is MRI 2.6.4 has a severe String corruption bug, so that might cause people to stay on 2.6.3 until 2.6.5 is released with the fix.

[nirvdrum]

Thanks. I'm fairly well aware of the issues with trying to share the Gemfile. I probably wouldn't have even tried MRI if I hadn't run into #1750. Thanks for the info on the String corruption bug, though. I hadn't seen that.

[nirvdrum]

FYI, MRI 2.6.5 was released on 01-Oct-2019.

[eregon]

Indeed, we're currently planning to update to 2.6.5. I updated the issue title as well.

[eregon]

We updated to Ruby 2.6.5 in 9b8b857 and restored the RDoc template files.