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>

bulma/layout/header.sass

@@ -36,7 +36,7 @@
     max-height: 24px
   a
     color: $text
-    flex: 1
+    flex-grow: 1
     &:hover
       color: $link-hover
     &.is-active
@@ -143,4 +143,3 @@
     &:hover,
     &.is-active
       border-bottom-width: 2px
-

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 {