fix(ionItem): vertically center select in an item

Closes #3517

Author: mhartington

scss/_select.scss

@@ -11,6 +11,7 @@
     @include appearance(none);
     position: absolute;
     top: 0;
+    bottom: 0;
     right: 0;
     padding: ($item-padding - 2) ($item-padding * 3) ($item-padding) $item-padding;
     max-width: 65%;