Pack less files in released gem

[JuanitoFatas]

Rationale: rubygems/bundler#3207

• don't pack hidden files

• don't pack test files (spec/*)

• don't pack bin/*

• pack these git tracking files

  > `git ls-files -z`.split("\x0").reject { |f| f.start_with?(".") || f =~ %r(^(spec|bin)/) }
  => ["Gemfile",
  "LICENSE.txt",
  "README.md",
  "Rakefile",
  "lib/uber.rb",
  "lib/uber/api.rb",
  "lib/uber/api/activities.rb",
  "lib/uber/api/me.rb",
  "lib/uber/api/price_estimates.rb",
  "lib/uber/api/products.rb",
  "lib/uber/api/promotions.rb",
  "lib/uber/api/requests.rb",
  "lib/uber/api/time_estimates.rb",
  "lib/uber/api_request.rb",
  "lib/uber/arguments.rb",
  "lib/uber/base.rb",
  "lib/uber/client.rb",
  "lib/uber/error.rb",
  "lib/uber/models/activity.rb",
  "lib/uber/models/estimate.rb",
  "lib/uber/models/map.rb",
  "lib/uber/models/price.rb",
  "lib/uber/models/product.rb",
  "lib/uber/models/promotion.rb",
  "lib/uber/models/request.rb",
  "lib/uber/models/time.rb",
  "lib/uber/models/user.rb",
  "lib/uber/parse_json.rb",
  "lib/uber/rate_limit.rb",
  "lib/uber/token.rb",
  "lib/uber/utils.rb",
  "lib/uber/version.rb",
  "uber-sdk.gemspec"]

[chrisenytc]

Thank you @JuanitoFatas