Bump version to 1.4.2

[st0012]

Since there's been a couple of crashing fixes merged after v1.4.1, like:

• IRB raises exception when stdout is a pipe #353
• Change to explicit method call in completion #369
• Fix colorize_code, fix irb crash. #391
• Handle non-String $LOAD_PATH values more carefully #400

I think it'd be great if we can cut a patch release for them.

From the diff between current master and v1.4.1, all PRs are either fixes, refactorings, or document updates.

[st0012]

@hsbt Could you help me cut this release? 🙏
irb doesn't have a changelog nor make release notes, so I don't prepare anything for them. But if you think it's time to start providing a changelog, I'll add one to this PR.

(I've consulted @k0kubun about this release)

[hsbt]

I will handle this today.

[hsbt]

done https://github.com/ruby/irb/releases/tag/v1.4.2

[st0012]

@hsbt Thank you 🙌