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

Gem::Specification.new do |spec|
  spec.name          = "middleman-webp"
  spec.version       = Middleman::Webp::VERSION
  spec.authors       = ["Juhamatti Niemelä"]
  spec.email         = ["iiska@iki.fi"]
  spec.summary       = %q{WebP image conversion for Middleman}
  spec.description   = %q{Generate WebP versions of each image used in Middleman site during build.}
  spec.homepage      = "http://github.com/iiska/middleman-webp"
  spec.license       = "MIT"

  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"]

  spec.add_dependency "middleman-core", "~> 4.0.0"

  spec.add_development_dependency "bundler", "~> 1.5"
  spec.add_development_dependency "middleman-cli", "~> 4.0.0"
  spec.add_development_dependency "rake"
  #spec.add_development_dependency "simplecov"
  spec.add_development_dependency "mocha"
end
```

Declare dependency according to Middleman docs
user
```ruby
# coding: utf-8
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'middleman-webp/version'

Gem::Specification.new do |spec|
  spec.name          = "middleman-webp"
  spec.version       = Middleman::Webp::VERSION
  spec.authors       = ["Juhamatti Niemelä"]
  spec.email         = ["iiska@iki.fi"]
  spec.summary       = %q{WebP image conversion for Middleman}
  spec.description   = %q{Generate WebP versions of each image used in Middleman site during build.}
  spec.homepage      = "http://github.com/iiska/middleman-webp"
  spec.license       = "MIT"

  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"]

  spec.add_dependency "middleman-core", "~> 4.0"

  spec.add_development_dependency "bundler", "~> 1.5"
  spec.add_development_dependency "middleman-cli", "~> 4.0.0"
  spec.add_development_dependency "rake"
  #spec.add_development_dependency "simplecov"
  spec.add_development_dependency "mocha"
end
```
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