colorbox: Upgraded module from version 7.x-2.3 to version 7.x-2.4.

Important: When updating to this version of the Colorbox module you need to update the Colorbox plugin to v1.3.21.1 or later as well.

Bug fixes:

    #1910162: "A script on this page is causing Internet Explorer to run slowly" error in IE7
    #1921636: Stop Caption from sliding down, or keep it up longer
    #1925056: Built in styles Style - Issue with styling on <button> in recent versions of the Colorbox plugin
    #1955982: Library pattern not match for master branch
    #1964812: in-module documentation still references deprecated form functionality

Misc:

    Improved the installations instructions in the README.