Updated Heroku deployment guide

fixes #sinatra/sinatra-book/sinatra#62

book/Deployment.markdown

@@ -11,32 +11,43 @@ pushing to a remote git repository.
 Steps to deploy to Heroku:
 
 * Create an [account](http://heroku.com/signup) if you don't have one
-* `gem install heroku`
+* Download and install [Heroku toolbelt](https://toolbelt.heroku.com/)
+* Make sure you have a `Gemfile`. Otherwise, you can create one and install the `sinatra` gem using `bundler`.
 * Make a config.ru in the root-directory
 * Create the app on heroku
 * Push to it
 
-1. Here is an example config.ru file that does two things.  First, it requires
+1. Install `bundler` if you haven't yet (`gem install bundler`). Create a `Gemfile` using `bundle init`. Modify your `Gemfile` to look like:
+
+    ```ruby
+    source 'http://rubygems.org'
+
+    gem 'sinatra'
+    ```
+
+    Run `bundle` to install the gem.
+
+    It is possible to specify a specific Ruby version on your Gemfile.
+    For more information, check [this](https://devcenter.heroku.com/articles/ruby-versions).
+
+2. Here is an example config.ru file that does two things.  First, it requires
    your main app file, whatever it's called. In the example, it will look for
-   `myapp.rb`.  Second, run your application.  If you're subclassing, use the
+   `myapp.rb`. Second, run your application. If you're subclassing, use the
    subclass's name, otherwise use Sinatra::Application.
 
-```ruby
-require "myapp"
-
-run Sinatra::Application
-```
+    ```ruby
+    require "myapp"
 
-2. Create the app and push to it
+    run Sinatra::Application
+    ```
 
-       From the root-directory of the application
+3. Create the app and push to it
 
-```
-$ heroku create <app-name>  # This will add heroku as a remote
-$ git push heroku master
-```
+   From the root directory of the application, run these commands:
 
-For more details see [this](http://github.com/sinatra/heroku-sinatra-app)
+    $ heroku create <app-name>  # This will add heroku as a remote
+    $ git push heroku master
 
-[Heroku]: http://www.heroku.com
+  For more details see [this](http://github.com/sinatra/heroku-sinatra-app).
 
+  [Heroku]: http://www.heroku.com