RubyGems 3.0.0 (en)

Major enhancements:

• S3 source. Pull request #1690 by Aditya Prakash.
• Download gems with threads. Pull request #1898 by André Arko.
• Update to SPDX license list 3.0. Pull request #2152 by Mike Linksvayer.
• [GSoC] Multi-factor feature for RubyGems. Pull request #2369 by Qiu -Chaofan.
• Use bundler 1.17.2. Pull request #2521 by SHIBATA Hiroshi.