update gantry to v4.1.15

Release Notes - Gantry Joomla Framework - Version 4.1.15

Bug
• add support for font awesome 3.2.1

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NOTE: Support for 3.2.1 is not automatic, due to major changes in Font Awesome, so in order to ensure backward compatibility, a manual change is required in order to add 3.2.1 to your Gantry based template. Please test throughly as some icons may not work as before.

In YOUR_SITE/templates/YOUR_TEMPLATE/less/global.less find:

     @import "jui/less/font-awesome.less";

And change to:

     @import "jui/less/font-awesome/font-awesome.less";