ruby: add rubygems customization, fix postinstall

This configures rubygems to install all gems into a public, non-versioned
directory by default - both gems and their binaries. (This means that
users no longer need /usr/local/opt/ruby/bin in their PATH.)

This means that we no longer have to monkey with gem directories at all
in postinstall. Bundled gems will be symlinked into this location.

Fixes Homebrew#29982.
Closes Homebrew#29986.

Library/Formula/ruby.rb

@@ -4,7 +4,7 @@ class Ruby < Formula
   homepage 'https://www.ruby-lang.org/'
   url "http://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.2.tar.bz2"
   sha256 "6948b02570cdfb89a8313675d4aa665405900e27423db408401473f30fc6e901"
-  revision 1
+  revision 2
 
   bottle do
     sha1 "9feba2200305e8750c26f83d562b900d31978905" => :mavericks
@@ -60,14 +60,19 @@ def install
     system "./configure", *args
     system "make"
     system "make install"
+
+    # Customize rubygems to look/install in the global gem directory
+    # instead of in the Cellar, making gems last across reinstalls
+    (lib/"ruby/2.1.0/rubygems/defaults/operating_system.rb").write rubygems_config
   end
 
   def post_install
-    # Preserve gem, site, and vendor folders on upgrade/reinstall
-    # by placing them in HOMEBREW_PREFIX and sym-linking
+    # Preserve site and vendor folders on upgrade/reinstall
+    # by placing them in HOMEBREW_PREFIX and symlinking
     ruby_lib = HOMEBREW_PREFIX/"lib/ruby"
 
-    ["gems", "site_ruby", "vendor_ruby"].each do |name|
+    # These directories are empty on install
+    ["site_ruby", "vendor_ruby"].each do |name|
       link = lib/"ruby"/name
       real = ruby_lib/name
 
@@ -76,18 +81,61 @@ def post_install
       real.mkpath
 
       link.unlink if link.exist?
-      link.symlink real
+      link.make_symlink real
     end
   end
 
-  def caveats; <<-EOS.undent
-    By default, gem installed executables will be placed into:
-      #{opt_bin}
-
-    You may want to add this to your PATH. After upgrades, you can run
-      gem pristine --all --only-executables
-
-    to restore binstubs for installed gems.
+  def rubygems_config; <<-EOS.undent
+    module Gem
+      def self.default_dir
+        path = [
+          "#{HOMEBREW_PREFIX}",
+          "lib",
+          "ruby",
+          "gems",
+          "2.1.0"
+        ]
+
+        @default_dir ||= File.join(*path)
+      end
+
+      def self.private_dir
+        path = if defined? RUBY_FRAMEWORK_VERSION then
+                 [
+                   File.dirname(RbConfig::CONFIG['sitedir']),
+                   'Gems',
+                   RbConfig::CONFIG['ruby_version']
+                 ]
+               elsif RbConfig::CONFIG['rubylibprefix'] then
+                 [
+                  RbConfig::CONFIG['rubylibprefix'],
+                  'gems',
+                  RbConfig::CONFIG['ruby_version']
+                 ]
+               else
+                 [
+                   RbConfig::CONFIG['libdir'],
+                   ruby_engine,
+                   'gems',
+                   RbConfig::CONFIG['ruby_version']
+                 ]
+               end
+
+        @private_dir ||= File.join(*path)
+      end
+
+      def self.default_path
+        if Gem.user_home && File.exist?(Gem.user_home)
+          [user_dir, default_dir, private_dir]
+        else
+          [default_dir, private_dir]
+        end
+      end
+
+      def self.default_bindir
+        "#{HOMEBREW_PREFIX}/bin"
+      end
+    end
     EOS
   end