fix dependency warning

[tuwilof]

gem build esplanade.gemspec 
WARNING:  no homepage specified
WARNING:  open-ended dependency on byebug (>= 10.0.0, development) is not recommended
  if byebug is semantically versioned, use:
    add_development_dependency 'byebug', '~> 10.0', '>= 10.0.0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: esplanade
  Version: 1.6.0
  File: esplanade-1.6.0.gem