Use box-sizing SCSS mixin rather than property

lms/static/sass/base/_base.scss

@@ -107,7 +107,7 @@ a:focus {
 
 .container {
   @include clearfix;
-  box-sizing: border-box;
+  @include box-sizing(border-box);
   margin: 0 auto 0;
   padding: 0px 30px;
   max-width: grid-width(12);

lms/static/sass/shared/_footer.scss

@@ -8,7 +8,7 @@
 
   footer {
     @include clearfix();
-    box-sizing: border-box;
+    @include box-sizing(border-box);
     max-width: grid-width(12);
     min-width: 760px;
     width: flex-grid(12);