specification: dependencies' PORO format and json format

[sanemat]

   s.dependencies = [Gem::Dependency.new("i18n", Gem::Requirement.new(["~> 0.7"]), :runtime),
    Gem::Dependency.new("json", Gem::Requirement.new([">= 1.7.7", "~> 1.7"]), :runtime),
    Gem::Dependency.new("tzinfo", Gem::Requirement.new(["~> 1.1"]), :runtime),
    Gem::Dependency.new("minitest", Gem::Requirement.new(["~> 5.1"]), :runtime),
    Gem::Dependency.new("thread_safe", Gem::Requirement.new([">= 0.3.4", "~> 0.3"]), :runtime)]