Fixes #131: header items broken on IE11

When using the shorthand property on .header-item
the flex-basis is set to 0px in IE11

Signed-off-by: Deyan Yanakiev <kliomba@abv.bg>

css/bulma.css

@@ -3990,9 +3990,9 @@ a.menu-block:hover {
 .header-item a {
   color: #69707a;
   -webkit-box-flex: 1;
-  -webkit-flex: 1;
-      -ms-flex: 1;
-          flex: 1;
+  -webkit-flex-grow: 1;
+      -ms-flex-positive: 1;
+          flex-grow: 1;
 }
 
 .header-item a:hover {