use verticalMargin when calculating min height in rows

Testing:
1. run two.html and check that yellow grid has height of 300px
2. modify two.html to have
#grid1 {
  background: lightgoldenrodyellow;
   min-height: 300px;
}
3. rerun and notice grid1 now has 380px actual height (passed last item) !
4. adding removing items will shrink grow, but not to the correct min 300px size.

package.json

@@ -9,7 +9,8 @@
   },
   "scripts": {
     "build": "grunt ; doctoc ./README.md ; doctoc ./doc/README.md ; doctoc ./doc/CHANGES.md",
-    "test": "grunt lint && karma start karma.conf.js"
+    "test": "grunt lint && karma start karma.conf.js",
+    "lint": "grunt lint"
   },
   "keywords": [
     "gridstack",

src/gridstack.js

@@ -1033,10 +1033,14 @@
             return;
         }
         var height = this.grid.getGridHeight();
-        var minHeight = parseInt(this.container.css('min-height')) / this.cellHeight();
+        // check for css min height. Each row is cellHeight + verticalMargin, until last one which has no margin below
+        var cssMinHeight = parseInt(this.container.css('min-height'));
+        if (cssMinHeight > 0) {
+            var minHeight = (cssMinHeight + this.opts.verticalMargin) / (this.cellHeight() + this.opts.verticalMargin);
             if (height < minHeight) {
                 height = minHeight;
             }
+        }
         this.container.attr('data-gs-current-height', height);
         if (!this.opts.cellHeight) {
             return ;