Fix mixed content errors

[0b10011]

Currently Firefox reports the following errors (Chrome reports similar errors):

• Blocked loading mixed active content "http://code.jquery.com/jquery-1.10.1.min.js"[Learn More] validator-demo
• Blocked loading mixed active content "http://ghbtns.com/github-btn.html?user=mailgun&repo=validator-demo&type=watch&size=large&count=true"[Learn More] validator-demo
• Blocked loading mixed active content "http://ghbtns.com/github-btn.html?user=mailgun&repo=validator-demo&type=fork&size=large&count=true"[Learn More] validator-demo
• ReferenceError: $ is not defined mailgun_validator.js:31:0
• ReferenceError: $ is not defined validator-demo:49:6

Using // instead of http:// lets the browser inherit the protocol from the main page. Alternatively, you could specify https://.

[jeremyschlatter]

Thanks!