VerticallMenu On IE 11 is broken

[Kim Jongho]

Description

On Your Site(http://mm.onokumus.com/mm-vertical.html)
VericalMenu on IE 11 is broken.
Look at the below image and it's sidebar.

Error message

[Kim Jongho]

For few minutes digging, if found "display:flex;" cause a problem in IE 11.
Without flex & -ms-flexbox , it works on IE11,IE Edge, FF, Chrome.
But not tested on IE 8,9 etc...

.sidebar-nav .metismenu {
    background: #212529;
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -webkit-box-orient: vertical;
    -webkit-box-direction: normal;
    -ms-flex-direction: column;
    flex-direction: column;
}