Update rainbow requirement from ~> 2.1 to >= 2.1, < 4.0

[dependabot-preview]

Updates the requirements on rainbow to permit the latest version.

Changelog

Sourced from rainbow's changelog.

2.2.2 (2017-04-21)

• added explicit rake dependency to fix installation issue

2.2.1 (2016-12-28)

• fixed gem installation (2.2.0 was a broken release)

2.2.0 (2016-12-27)

• improved Windows support
• added Ruby 2.4 support
• added bold alias method for bright

2.1.0 (2016-01-24)

• added X11 color support
• fixed require issue when rainbow is used as a dependency in another gem
• improved Windows support

2.0.0 (2014-01-24)

• disable string mixin by default

1.99.2 (2014-01-24)

• bring back ruby 1.8 support

1.99.1 (2013-12-28)

• drop support for ruby 1.8
• require "rainbow/string" -> require "rainbow/ext/string"
• custom rainbow wrapper instances (with separate enabled/disabled state)
• shortcut methods for changing text color (Rainbow("foo").red)

1.99.0 (2013-12-26)

• preparation for dropping String monkey patching
• require "rainbow/string" if you want to use monkey patched String
• introduction of Rainbow() wrapper
• support for MRI 1.8.7, 1.9.2, 1.9.3, 2.0 and 2.1, JRuby and Rubinius
• deprecation of Sickill::Rainbow namespace (use Rainbow.enabled = true instead)

1.1.4 (2012-4-28)

• option for forcing coloring even when STDOUT is not a TTY (CLICOLOR_FORCE env var)
• fix for frozen strings

1.1.3 (2011-12-6)

... (truncated)

Commits

• 75a1f87 Bump version
• 5fe5e69 Merge pull request #58 from mattrayner/mattrayner/add-rake-as-gem-dependency
• 16c7969 Re-wrap color names
• 276014e Add open-ended rake dependency
• 3e8064c Merge pull request #53 from ypresto/add-faint
• e364abd Add faint/dark method
• 21f3529 Fix missing bold alias in null presenter
• f9785ab Merge pull request #52 from syndbg/master
• f8c01d9 Fix Color.build ignoring Integers in older Ruby versions
• b27e615 Bump version and update changelog
• Additional commits viewable in compare view

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