title: :blank selector not working on checkbox jQuery Validate()
body: <p>Hi I have currently have 2 fields and either 1 or the other need to be checked, these were initially using 2 text type inputs, now 1 of them needs to change from a text input to a checkbox.</p>
  
  <p>Logic: IF the textbox has a value, checkbox is not required, 
  IF checkbox is ticked, textbox is not required.</p>
  
  <p>I was initially using the <code>:blank</code> selector to determine the validation and it was working, changed it to a checkbox and it broke, is there a different rule for checkbox's that I am missing from the validate() docs?</p>
  
  <p>I am using the following versions:</p>
  
  <pre><code>&lt;script src="//cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.13.1/jquery.validate.js"&gt;&lt;/script&gt;
  &lt;script src="//cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.13.1/additional-methods.js"&gt;&lt;/script&gt;
  </code></pre>
  
  <p>jQuery:</p>
  
  <pre><code>        //text 1
          'textbox': {
              required: "#certificate:blank",
              minlength: 8
          },
  
          //checkbox 1
          certificate: {
              required: "#textbox:blank"
          },
  </code></pre>
  
  <p>HTML:</p>
  
  <pre><code>&lt;input type="text" maxlength="9" class=" form-textbox" id="textbox" name="textbox"&gt;
  &lt;input type="checkbox" class=" form-checkbox" id="certificate" name="certificate"&gt;
  </code></pre>