roster.rb

def skills_in_roster(roster)
  skills = roster.map { |_, skills_array| skills_array }.flatten.uniq
end

def developers_with_skill(skill, roster)
  roster.select do |developer, developer_skills|
    developer_skills.include?(skill)
  end.
    keys.map(&:to_s)
end

def skill_map(roster)
  Hash[
    skills_in_roster(roster).map do |skill|
      [skill, developers_with_skill(skill, roster)]
    end
  ]
end