```ruby
# coding: utf-8
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'bagit/version'

Gem::Specification.new do |spec|
  spec.name        = "bagit"
  spec.version     = BagIt::VERSION
  spec.summary     = "BagIt package generation and validation"
  spec.description = "Ruby Library and Command Line tools for bagit"
  spec.email       = "jamie@jamielittle.org"
  spec.homepage    = 'http://github.com/tipr/bagit'
  spec.authors     = ["Tom Johnson, Francesco Lazzarino, Jamie Little"]
  spec.license     = "MIT"

  spec.add_dependency 'validatable', '~> 1.6'
  spec.add_dependency 'docopt', '~> 0.5.0'

  spec.add_development_dependency 'bundler'
  spec.add_development_dependency 'rake', '~> 10.4'
  spec.add_development_dependency 'rspec', '~> 3'
  spec.add_development_dependency 'coveralls'

  spec.files         = `git ls-files`.split($/)
  spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
  spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
  spec.require_paths = ["lib"]
end
```

Update gemspec with min ruby version
user
```ruby
# coding: utf-8
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'bagit/version'

Gem::Specification.new do |spec|
  spec.name        = "bagit"
  spec.version     = BagIt::VERSION
  spec.summary     = "BagIt package generation and validation"
  spec.description = "Ruby Library and Command Line tools for bagit"
  spec.email       = "jamie@jamielittle.org"
  spec.homepage    = 'http://github.com/tipr/bagit'
  spec.authors     = ["Tom Johnson, Francesco Lazzarino, Jamie Little"]
  spec.license     = "MIT"

  spec.required_ruby_version = '~> 2.0'
  
  spec.add_dependency 'validatable', '~> 1.6'
  spec.add_dependency 'docopt', '~> 0.5.0'

  spec.add_development_dependency 'bundler'
  spec.add_development_dependency 'rake', '~> 10.4'
  spec.add_development_dependency 'rspec', '~> 3'
  spec.add_development_dependency 'coveralls'

  spec.files         = `git ls-files`.split($/)
  spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
  spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
  spec.require_paths = ["lib"]
end
```
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