Remove jruby-specific platform

[kfcampbell]

Resolves #1519. Thank you @composerinteralia for the tip!

Previously, our Ruby head build was failing looking for a specific version of json in a list of empty platforms. Speculative: perhaps an underlying change in Ruby or bundler caused errors with the platforms: [:jruby] specification in our Gemfile.

---

Behavior

Before the change?

• Ruby head builds are broken due to a gem resolution error.

After the change?

• Ruby head builds are successful.

Other information

• This issue is obnoxious! We want to fix the build so we can ship several PRs we have on deck.

---

Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

---

[composerinteralia]

rubygems/rubygems#5960 is what changed the behavior

[nickfloyd]

💥