Ship default .rubocop.yml

Currently, there is no `.rubocop.yml` shipped by default.
So when a user runs `rubocop` after creating a new gem via
`bundle gem foo`, it throws a bunch of offenses.

With the default `.rubocop.yml` present, the number of those
offenses significantly reduce by 25.

Signed-off-by: Utkarsh Gupta <utkarsh@debian.org>

lib/bundler/cli/gem.rb

@@ -79,6 +79,7 @@ module Bundler
       ]
 
       templates.merge!("gitignore.tt" => ".gitignore") if Bundler.git_present?
+      templates.merge!("rubocop.yml.tt" => ".rubocop.yml")
 
       if test_framework = ask_and_set_test_framework
         config[:test] = test_framework

lib/bundler/templates/newgem/rubocop.yml.tt

@@ -0,0 +1,7 @@
+Style/StringLiterals:
+  Enabled: true
+  EnforcedStyle: double_quotes
+
+Style/StringLiteralsInInterpolation:
+  Enabled: true
+  EnforcedStyle: double_quotes

spec/bundler/commands/newgem_spec.rb

@@ -326,6 +326,7 @@ RSpec.describe "bundle gem" do
       expect(bundled_app("#{gem_name}/lib/#{require_path}.rb")).to exist
       expect(bundled_app("#{gem_name}/lib/#{require_path}/version.rb")).to exist
       expect(bundled_app("#{gem_name}/.gitignore")).to exist
+      expect(bundled_app("#{gem_name}/.rubocop.yml")).to exist
 
       expect(bundled_app("#{gem_name}/bin/setup")).to exist
       expect(bundled_app("#{gem_name}/bin/console")).to exist
@@ -384,6 +385,12 @@ RSpec.describe "bundle gem" do
       expect(bundled_app("#{gem_name}/lib/#{require_path}.rb").read).to match(/class Error < StandardError; end$/)
     end
 
+    it "creates a default .rubocop.yml" do
+      bundle "gem #{gem_name}"
+
+      expect(bundled_app("#{gem_name}/.rubocop.yml")).to exist
+    end
+
     it "runs rake without problems" do
       bundle "gem #{gem_name}"