title: New variable stripe_subscription required for Cashier with Laravel 5
body: <p>I'm upgrading from Laravel 4.2 to Laravel 5.1.  I have a Stripe account that works with Cashier in my Laravel 4.2 version that I built watching a Laracast on Stripe and Cashier.  However, in Laravel 5.1 a new table column is required called "stripe_subscription."  I tried researching the Stripe documentation, the Cashier documentation and the Laravel documentation, but couldn't find anything about what this variable is or does.  </p>
  
  <p>After simply adding it to my database with a new migration, it looks like Stripe is up and running again with Cashier in my Laravel 5.1 version.  However, I was wondering if anyone knows what this variable does or where I can learn more about it.  I just guessed that it should be a string, which I was really unhappy about having to guess, but it seemed to work.  </p>
  
  <p>Here is some code relevant to this variable from my project:</p>
  
  <p>From my migration:          </p>
  
  <pre><code>$table-&gt;string('stripe_subscription')-&gt;nullable();
  </code></pre>
  
  <p>From the Cashier file Billable.php:</p>
  
  <pre><code> public function setStripeSubscription($subscription_id)
      {
          $this-&gt;stripe_subscription = $subscription_id;
  
          return $this;
      }
  
  
  public function getStripeSubscription()
      {
          return $this-&gt;stripe_subscription;
      }
  
  public function deactivateStripe()
      {
          $this-&gt;setStripeIsActive(false);
  
          $this-&gt;stripe_subscription = null;
  
          return $this;
      }
  </code></pre>
  
  <p>I'm just noticing it does actually say that stripe_subscription is a string in the php file, but I'd rather see something in formal documentation about it.  Further, as far as I can tell, this variable is unnecessary because there is also a "stripe_plan" variable.  The "stripe_subscription" variable contains a random string, while the "stripe_plan" variable is the name of the plan, like "monthly" or "yearly."  </p>
  
  <p>If anyone can give me some guidance on where I can learn more about this variable and how to work with it, I'd appreciate it.</p>