Update bundler requirement from ~> 2.1.4 to ~> 2.2.15

[dependabot-preview[bot]]

Updates the requirements on bundler to permit the latest version.

Release notes

Sourced from bundler's releases.

bundler-v2.2.15

Enhancements:

• Add a hint about bundler installing executables for path gems #4461
• Warn lockfiles with incorrect resolutions #4459
• Don't generate duplicate redundant sources in the lockfile #4456

Bug fixes:

• Respect running ruby when resolving platforms #4449

Changelog

Sourced from bundler's changelog.

2.2.15 (March 19, 2021)

Enhancements:

• Add a hint about bundler installing executables for path gems #4461
• Warn lockfiles with incorrect resolutions #4459
• Don't generate duplicate redundant sources in the lockfile #4456

Bug fixes:

• Respect running ruby when resolving platforms #4449

2.2.14 (March 8, 2021)

Security fixes:

• Lock GEM sources separately and fix locally installed specs confusing bundler #4381

Bug fixes:

• Make rake available to other gems' installers right after it's installed #4428
• Fix encoding issue on compact index updater #4362

2.2.13 (March 3, 2021)

Enhancements:

• Respect user configured default branch in README links in new generated gems #4303

Bug fixes:

• Fix gems sometimes being pulled from irrelevant sources #4418

2.2.12 (March 1, 2021)

Bug fixes:

• Fix sporadic warnings about nil gemspec on install/update and make those faster #4409
• Fix deployment install with duplicate path gems added to Gemfile #4410

2.2.11 (February 17, 2021)

Bug fixes:

• Revert disable_multisource changes #4385

2.2.10 (February 15, 2021)

Security fixes:

... (truncated)

Commits

• 3dbcc68 Merge pull request #4470 from rubygems/release/bundler_2.2.15_rubygems_3.2.15
• 30438af Bump Rubygems version to 3.2.15
• 8a5639f Changelog for Rubygems version 3.2.15
• 27055e8 Bump Bundler version to 2.2.15
• 9bbbdf5 Changelog for Bundler version 2.2.15
• f04f81d Merge pull request #4461 from rubygems/improve_using_message_for_path_gems
• 1d5453c Merge pull request #4464 from sigstore/fix-for-4463
• 206df19 Merge pull request #4459 from rubygems/fail_lockfiles_with_incorrect_resolutions
• 4d5b85a Merge pull request #4460 from rubygems/prevent_downgrading_too_much
• ccec74e Merge pull request #4456 from rubygems/redundant_sources
• Additional commits viewable in compare view

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