Replace style guide with one based on Thoughtbot's

Copy metrics cops settings and some other most basic configuration to
.rubocop.ribose.yml, and inherit everything else from Thoughtbot's
style guide.

Replace old, unmanageable monolithic style guide with a versioned
reference to our style guide.

style/.rubocop.ribose.yml

@@ -0,0 +1,67 @@
+inherit_from:
+  - https://raw.githubusercontent.com/thoughtbot/guides/91509c26b3f84beebb239592561e41a77fdd0fa6/style/ruby/.rubocop.yml
+AllCops:
+  Include:
+  - "**/*.rake"
+  - "**/Gemfile"
+  - "**/*.gemfile"
+  - "**/Rakefile"
+  Exclude:
+  - "vendor/**/*"
+  - "db/**/*"
+  - "tmp/**/*"
+  DisplayCopNames: false
+  StyleGuideCopsOnly: false
+Rails:
+  Enabled: true
+Metrics/AbcSize:
+  Description: A calculated magnitude based on number of assignments, branches, and
+    conditions.
+  Enabled: true
+  Max: 15
+Metrics/BlockLength:
+  Exclude:
+    - "spec/**/*"
+Metrics/BlockNesting:
+  Description: Avoid excessive block nesting
+  StyleGuide: https://github.com/bbatsov/ruby-style-guide#three-is-the-number-thou-shalt-count
+  Enabled: true
+  Max: 3
+Metrics/ClassLength:
+  Description: Avoid classes longer than 100 lines of code.
+  Enabled: false
+  CountComments: false
+  Max: 100
+Metrics/CyclomaticComplexity:
+  Description: A complexity metric that is strongly correlated to the number of test
+    cases needed to validate a method.
+  Enabled: true
+  Max: 6
+Metrics/LineLength:
+  Description: Limit lines to 80 characters.
+  StyleGuide: https://github.com/bbatsov/ruby-style-guide#80-character-limits
+  Enabled: true
+  Max: 80
+  AllowURI: true
+  URISchemes:
+  - http
+  - https
+Metrics/MethodLength:
+  Description: Avoid methods longer than 10 lines of code.
+  StyleGuide: https://github.com/bbatsov/ruby-style-guide#short-methods
+  Enabled: true
+  CountComments: true
+  Max: 10
+  Exclude:
+  - "spec/**/*"
+Metrics/ParameterLists:
+  Description: Avoid long parameter lists.
+  StyleGuide: https://github.com/bbatsov/ruby-style-guide#too-many-params
+  Enabled: true
+  Max: 5
+  CountKeywordArgs: true
+Metrics/PerceivedComplexity:
+  Description: A complexity metric geared towards measuring complexity for a human
+    reader.
+  Enabled: true
+  Max: 7