Fix for https detection

Please pull.

I have tested my code against PHP 5.5, 5.4 and 5.3.

you can see the results on my travis page for this repo

https://travis-ci.org/SEOVictory/wistia-wordpress-plugin

You can see from the offical php documents how https should be detected from the $_SERVER variable

http://php.net/manual/en/reserved.variables.server.php

'HTTPS'
Set to a non-empty value if the script was queried through the HTTPS protocol.
Note: Note that when using ISAPI with IIS, the value will be off if the request was not made through the HTTPS protocol.

The current version of wistia-wordpress-plugin results in a notice error thrown. This is bad because it will be filling up logs or showing up to the end user on http pages. It could also cause http requests when https requests are needed on servers who set $_SERVER['HTTPS'] to something other than 'On' which is allowed by php language as per the official documentation.

Thank you!