Update rdoc requirement from ~> 4.1 to ~> 6.4

[dependabot[bot]]

Updates the requirements on rdoc to permit the latest version.

Changelog

Sourced from rdoc's changelog.

=== 5.1.0 / 2017-02-24

• Bug fixes

  • Fix an issue that rdoc fails when running on Windows with RUBYOPT=-U. PR #430 by Toshihiko Ichida

• Minor enhancements

  • Parse ruby 2.1 def. PR #436 by Akira Matsuda.
  • Suppress warnings in eval. PR #440 by Nobuyoshi Nakada.

=== 5.0.0 / 2016-11-05

• Major enhancements

  • Cleanup deprecated code targeted Ruby 1.8

• Bug fixes

  • Ensure badge data is included in result of JsonIndex template.
  • Ensure items in the nil section are displayed in HTML output. Issue #399 by Daniel Svensson.
  • Parse rb_intern_const correctly in C. PR #381 by Sho Hashimoto.
  • Fix broken assets caused by #335 when serving ri. PR #360 by Alex Wood.
  • Don't try to parse svg files. Issue #350 by Sigurd Svela.

• Minor enhancements

  • Improve class name expansion/resolution in ri. PR #400 by NARUSE, Yui
  • Improve performance of document generation. PR #397 by Yusuke Endoh.

=== 4.3.0 / 2016-11-04

• Minor enhancements
  • Removed json dependency for Ruby 2.4.0
  • End to support Ruby 1.8.x

=== 4.2.2 / 2016-02-09

• Bug fixes
  • Include lib/rdoc/generator/pot/* in built gem

=== 4.2.1 / 2015-12-22

• Bug fixes
  • Fixed infinite loop with CR #339 by @​nobu
  • Allow rdoc run with --disable-gems #340 by @​luizluca
  • Don't store full path in GZipped js files #341 by @​voxik
  • Fix relative path names for drive letters #367 by @​nobu
  • Fix for valid syntax class C end parsing #368 by @​nobu

=== 4.2.0 / 2014-12-06

... (truncated)

Commits

• 35b409b Version 6.4.0
• db954b7 Merge pull request #853 from aycabta/resolve-class-and-method-of-the-same-nam...
• 23747b4 Support addition Object class when adding methods to top level
• 1e16284 Resolve class and method of the same name correctly
• c81b52e Merge pull request #850 from nobu/typos
• 8386333 Fix typos in test method names [ci skip]
• c749f60 Merge pull request #840 from flavorjones/flavorjones/alias-call-seq-fix
• 6d98e9f Merge pull request #845 from flavorjones/flavorjones-link-equals-method
• 50f0270 Merge pull request #846 from flavorjones/flavorjones-support-category-in-c-fu...
• 836c829 Merge pull request #847 from flavorjones/flavorjones-fix-default-format-for-c...
• Additional commits viewable in compare view

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