Added missing !default in scss variables

Closes #2231

scss/_variables.scss

@@ -3,11 +3,11 @@
 
 $fa-font-path:        "../fonts" !default;
 //$fa-font-path:        "//netdna.bootstrapcdn.com/font-awesome/4.0.1/fonts" !default; // for referencing Bootstrap CDN font files directly
-$fa-css-prefix:       fa;
+$fa-css-prefix:       fa !default;
 $fa-version:          "4.0.1" !default;
 $fa-border-color:     #eee !default;
 $fa-inverse:          #fff !default;
-$fa-li-width:        (30em / 14);
+$fa-li-width:        (30em / 14) !default;
 
 $fa-var-glass: "\f000";
 $fa-var-music: "\f001";

src/assets/font-awesome/scss/_variables.scss

@@ -5,11 +5,11 @@
 
 $fa-font-path:        "../fonts" !default;
 //$fa-font-path:        "//netdna.bootstrapcdn.com/font-awesome/{{site.fontawesome.version}}/fonts" !default; // for referencing Bootstrap CDN font files directly
-$fa-css-prefix:       {{ site.fontawesome.css_prefix }};
+$fa-css-prefix:       {{ site.fontawesome.css_prefix }} !default;
 $fa-version:          "{{ site.fontawesome.version }}" !default;
 $fa-border-color:     #eee !default;
 $fa-inverse:          #fff !default;
-$fa-li-width:        (30em / 14);
+$fa-li-width:        (30em / 14) !default;
 
 {% for icon in icons %}$fa-var-{{ icon.id }}: "\{{ icon.unicode }}";
 {% endfor %}