Fixed icons problems when using some sass version

While using some sass version, the css output for icons may be something like
```css
.icon-note.icon-left a:before, .icon-note.icon-right a:after, i.icon-note:before {
    content: "\\1f3b5";
    height: inherit;
}
```
But not 
```css
.icon-note.icon-left a:before, .icon-note.icon-right a:after, i.icon-note:before {
    content: "\1f3b5";
    height: inherit;
}
```

So that icons will display with some unexpected chars. Simply add double  single quotation to solve this problem