v2.0.0

• Added graphina gem as a runtime dependency in Rakefile
• Replace internal Hackmac::Graph implementation with dependency on graphina v1.0.0
• Update bin/gfxmon to require graphina and use Graphina::Graph instead
  of Hackmac::Graph
• Migrate Hackmac::Graph::Formatters to Graphina::Graph::Formatters
• Pass true_coloring option directly to Graphina::Graph.new
• Remove ensure graph&.stop block as graphina handles cleanup internally
• Update lib/hackmac.rb to require graphina and remove hackmac/graph.rb
  require
• Delete old hackmac/graph.rb, hackmac/graph/display.rb,
  hackmac/graph/display/cell.rb, and hackmac/graph/formatters.rb files
• Remove default value for option s in gfxmon script
• Remove obsolete default value for sleep duration
• Update CHANGES.md with formatted bullet points