Uninitialized constant Magick::MaxRGB

[spalladino]

With rmagick 2.15.4, the following error is thrown when running sprite:

.../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/image_combiner.rb:21:in `composite_images': uninitialized constant Magick::MaxRGB (NameError)
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/builder.rb:106:in `block in write_image'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/builder.rb:93:in `each'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/builder.rb:93:in `write_image'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/builder.rb:35:in `block in build'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/builder.rb:34:in `each'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/builder.rb:34:in `build'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/lib/sprite/runner.rb:21:in `run!'
    from .../.bundle/ruby/2.2.0/gems/sprite-0.2.7/bin/sprite:8:in `<top (required)>'
    from .../.bundle/ruby/2.2.0/bin/sprite:23:in `load'
    from .../.bundle/ruby/2.2.0/bin/sprite:23:in `<main>'