Update ruby-rainbow to 1.99.1. Part of PR/48447.

Changelog (per Changelog.md)

## 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)

## 1.99.1 (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`)

(tsutsui)