ruby docs: improve the example

- missing cd command
- invoke bundler through nix-shell, so it doesn't need to be on $PATH
Note: running bundix through nix-shell won't work ATM, as the shell sets
SSL_CERT_FILE=/no-cert-file.crt which prevents fetching throug https.
- use version from gemset to simplify updating
- don't break line in meta.description

doc/languages-frameworks/ruby.xml

···
 <screen>
 <![CDATA[$ cd pkgs/servers/monitoring
 $ mkdir sensu
+$ cd sensu
 $ cat > Gemfile
 source 'https://rubygems.org'
 gem 'sensu'
-$ bundler package --path /tmp/vendor/bundle
+$ nix-shell -p bundler --command "bundler package --path /tmp/vendor/bundle"
 $ $(nix-build '<nixpkgs>' -A bundix)/bin/bundix
 $ cat > default.nix
 { lib, bundlerEnv, ruby }:
 
-bundlerEnv {
-  name = "sensu-0.17.1";
+bundlerEnv rec {
+  name = "sensu-${version}";
 
+  version = (import gemset).sensu.version;
   inherit ruby;
   gemfile = ./Gemfile;
   lockfile = ./Gemfile.lock;
   gemset = ./gemset.nix;
 
   meta = with lib; {
-    description = "A monitoring framework that aims to be simple, malleable,
-and scalable";
+    description = "A monitoring framework that aims to be simple, malleable, and scalable";
     homepage    = http://sensuapp.org/;
     license     = with licenses; mit;
     maintainers = with maintainers; [ theuni ];