Added an RSS Feed

Hi @dpmccabe,

I came across your excellent article _Recurring subscriptions with Ruby, RSpec and modular arithmetic_ from [Ruby Weekly](http://rubyweekly.com/) and wanted to subscribe to your site via RSS but did not find that option. This pull request is to add that option. 

In addition to the atom feed the `deploy.sh` script has been updated to ping Google's [PubSubHubbub](http://en.wikipedia.org/wiki/PubSubHubbub) hub when you deploy which will make for faster updates for RSS readers that support PuSH.

Here's a screenshot of what it will look like in a reader as well as the generated XML you would see with this template.

Thanks!

Ben

![screen shot 2014-01-16 at 4 08 49 pm](https://f.cloud.github.com/assets/133809/1936508/aba6a22c-7f0c-11e3-9265-6d2cbe83a052.png)

```xml
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <title>dmcca.be</title>
  <link href="http://dmcca.be/atom.xml" rel="self" />
  <link href="http://dmcca.be" />
  <link rel="hub" href="http://pubsubhubbub.appspot.com/" />
  <updated>2014-01-16T16:20:21-08:00</updated>
  <id>http://dmcca.be</id>

  
  <entry>
    <title>Recurring subscriptions with Ruby, RSpec and modular arithmetic</title>
    <author>
      <name>Devin McCabe</name>
    </author>
    <link href="http://dmcca.be/2014/01/09/recurring-subscriptions-with-ruby-rspec-and-modular-arithmetic.html" />
    <published>2014-01-09T08:19:50-08:00</published>
    
      <updated>2014-01-09T08:19:50-08:00</updated>
    
    <id>/2014/01/09/recurring-subscriptions-with-ruby-rspec-and-modular-arithmetic</id>
    <summary>Whether you&amp;#39;re developing the latest offering in the monthly subscription box space or simply adding a recurring subscription option to your e-commerce platform, you&amp;#39;ll likely find that figuring out how to model subscriptions isn&amp;#39;t the most obvious thing. Well, it&amp;#39;s not obvious unless you&amp;#39;ve come across modular arithmetic in a...</summary>
    <content type="html">&lt;p&gt;Whether you&amp;#39;re...</content>
  </entry>
  
</feed>
```