guix: build: ruby-build-system: Install to the vendor directory

* guix/build/ruby-build-system.scm (install): Install gems to the vendor
  directory, rather than the GEM_HOME. The vendor directory does not include
  the version of ruby used to install the gem in the path, which makes it
  easier to add it to the GEM_PATH for all versions of ruby to use.
  (gem-home): Remove procedure.
* gnu/packages/ruby.scm (ruby, ruby-2.1)[native-search-paths]: Switch to
  lib/ruby/vendor_ruby.
  (ruby-1.8)[native-search-paths]: Remove native-search-paths.
  (gem-directory): Remove procedure.
  (ruby-ansi, ruby-ae)[arguments]: Remove use of gem-directory.
  (ruby-metaclass, ruby-instantiator, ruby-introspection, ruby-mocha,
  ruby-nokogiri, ruby-minitest-tu-shim, ruby-redcloth)[arguments]: Remove use
  of gem-home.
  (ruby-git, ruby-httpclient)[arguments]: Remove use of GEM_HOME.
* gnu/packages/databases.scm (es-dump-restore)[arguments]: Remove use of
  GEM_HOME.