Update rdoc to `~> 4.2`

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2814

Gemfile

@@ -104,7 +104,7 @@ gem 'deckar01-task_list', '1.0.5', require: 'task_list/railtie'
 gem 'gitlab-markup',      '~> 1.5.0'
 gem 'redcarpet',          '~> 3.3.3'
 gem 'RedCloth',           '~> 4.3.2'
-gem 'rdoc',               '~>3.6'
+gem 'rdoc',               '~> 4.2'
 gem 'org-ruby',           '~> 0.9.12'
 gem 'creole',             '~> 0.5.0'
 gem 'wikicloth',          '0.8.1'

Gemfile.lock

@@ -567,7 +567,7 @@ GEM
       ffi (>= 0.5.0)
     rblineprof (0.3.6)
       debugger-ruby_core_source (~> 1.3)
-    rdoc (3.12.2)
+    rdoc (4.2.2)
       json (~> 1.4)
     recaptcha (3.0.0)
       json
@@ -933,7 +933,7 @@ DEPENDENCIES
   rails-deprecated_sanitizer (~> 1.0.3)
   rainbow (~> 2.1.0)
   rblineprof (~> 0.3.6)
-  rdoc (~> 3.6)
+  rdoc (~> 4.2)
   recaptcha (~> 3.0)
   redcarpet (~> 3.3.3)
   redis (~> 3.2)