Start using Bundler

- Add Gemfile
- Add auto-generated Gemfile.lock

Gemfile

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+# frozen_string_literal: true
+source "https://rubygems.org"
+
+gem "rubocop"
+gem "yard"
+gem "yard-ghpages"

Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    ast (2.3.0)
+    git (1.3.0)
+    parser (2.4.0.0)
+      ast (~> 2.2)
+    powerpack (0.1.1)
+    rainbow (2.2.2)
+      rake
+    rake (11.3.0)
+    rubocop (0.48.1)
+      parser (>= 2.3.3.1, < 3.0)
+      powerpack (~> 0.1)
+      rainbow (>= 1.99.1, < 3.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (~> 1.0, >= 1.0.1)
+    ruby-progressbar (1.8.1)
+    unicode-display_width (1.2.1)
+    yard (0.9.9)
+    yard-ghpages (0.0.2)
+      git (~> 1.3)
+      rake (~> 11.1)
+      yard (~> 0.8)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  rubocop
+  yard
+  yard-ghpages
+
+BUNDLED WITH
+   1.13.7