Fix up new margins on modals by not setting auto until screen-sm-min

dist/css/bootstrap.css

@@ -5899,8 +5899,6 @@ button.close {
   z-index: 1050;
   width: auto;
   margin: 10px;
-  margin-right: auto;
-  margin-left: auto;
 }
 
 .modal-content {
@@ -5998,8 +5996,7 @@ button.close {
 @media screen and (min-width: 768px) {
   .modal-dialog {
     width: 600px;
-    margin-top: 30px;
-    margin-bottom: 30px;
+    margin: 30px auto;
   }
   .modal-content {
     -webkit-box-shadow: 0 5px 15px rgba(0, 0, 0, 0.5);

less/modals.less

@@ -35,8 +35,6 @@
 // Shell div to position the modal with bottom padding
 .modal-dialog {
   position: relative;
-  margin-left: auto;
-  margin-right: auto;
   width: auto;
   margin: 10px;
   z-index: (@zindex-modal-background + 10);
@@ -122,8 +120,7 @@
 
   .modal-dialog {
     width: 600px;
-    margin-top: 30px;
-    margin-bottom: 30px;
+    margin: 30px auto;
   }
   .modal-content {
     .box-shadow(0 5px 15px rgba(0,0,0,.5));