Fix deprecated Rubygems options

In February 2013, Rubygems 2.0.0 deprecated [1] the `--[no-]ri` and `--[no-]rdoc` options, replacing them with the new `--[no-]document` option.

Almost five years later, Rubygems finally dropped support [2] for the long-deprecated `--[no-]ri` and `--[no-]rdoc` options.

This removal causes `bundle install` to **fail for fresh Homebrew checkouts** if Rubygems 3.0.x or newer is installed, which will happen if the user runs `gem update --system` on Ruby 2.3.0 or later. [3]

This also affects the system Ruby built into macOS 10.13 and 10.14.

Steps to reproduce:

```
$ sudo /usr/bin/gem update -N --system
[…]
Installing RubyGems 3.0.2
ERROR:  While executing gem ... (Errno::EPERM)
    Operation not permitted @ rb_sysopen - /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/gem
$ gem --version 2>/dev/null
3.0.2
$ cd "$(mktemp -d)"
$ git clone --depth=1 https://github.com/Homebrew/brew.git
[…]
$ cd brew
$ export HOMEBREW_PATH=bin  # in case shellcheck is not installed
$ bin/brew style Library/Homebrew/extend/ENV/std.rb
==> Installing or updating 'rubocop' gem
Error: invalid option: --no-ri
```

(Note: The above steps will not touch an existing Homebrew installation, and won’t even require Homebrew to be installed.)

This commit replaces the deprecated options with the new `--no-document` option, which has been around since Rubygems 2.0.0. It is therefore safe for use on all supported platforms.

[1]: https://blog.rubygems.org/2013/02/24/2.0.0-released.html
[2]: https://github.com/rubygems/rubygems/pull/2354
[3]: https://rubygems.org/gems/rubygems-update/versions

Library/Homebrew/utils.rb

@@ -221,7 +221,7 @@ module Homebrew
     return unless Gem::Specification.find_all_by_name(name, version).empty?
 
     ohai "Installing or updating '#{name}' gem"
-    install_args = %W[--no-ri --no-rdoc #{name}]
+    install_args = %W[--no-document #{name}]
     install_args << "--version" << version if version
 
     # Do `gem install [...]` without having to spawn a separate process or