RubyGem 'atk' が入らない

$ bundle update
で謎のエラーが出る。Linux Mint 18。

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/home/tomoki/Documents/Ruby/vendor/bundle/ruby/2.3.0/gems/atk-3.1.6/dependency-check
rake
RUBYARCHDIR=/home/tomoki/Documents/Ruby/vendor/bundle/ruby/2.3.0/extensions/x86_64-linux/2.3.0-static/atk-3.1.6
RUBYLIBDIR=/home/tomoki/Documents/Ruby/vendor/bundle/ruby/2.3.0/extensions/x86_64-linux/2.3.0-static/atk-3.1.6
/home/tomoki/.rbenv/versions/2.3.3/lib/ruby/2.3.0/rubygems.rb:241:in `bin_path':
can't find gem rake (>= 0.a) (Gem::GemNotFoundException)
	from /home/tomoki/.rbenv/versions/2.3.3/bin/rake:22:in `<main>'

rake failed, exit code 1

Gem files will remain installed in
/home/tomoki/Documents/Ruby/vendor/bundle/ruby/2.3.0/gems/atk-3.1.6 for
inspection.
Results logged to
/home/tomoki/Documents/Ruby/vendor/bundle/ruby/2.3.0/extensions/x86_64-linux/2.3.0-static/atk-3.1.6/gem_make.out

An error occurred while installing atk (3.1.6), and Bundler cannot
continue.
Make sure that `gem install atk -v '3.1.6'` succeeds before bundling.

In Gemfile:
  utils was resolved to 1.10.2, which depends on
    gtk2 was resolved to 3.1.6, which depends on
      atk

　
Gemfile の先頭に

gem 'rake'

を追加したら入った。何なんだ…。依存関係を記述していなかったのか？