Set a minimum height on the combobox button

to fix captionless button height.

IPython/html/static/notebook/less/widgets.less

@@ -98,6 +98,7 b''
 
         .widget-combo-btn {
             min-width: 138px; /* + 26px drop arrow btn = 164px */
+            min-height: 1ex;
         }
 
         .widget-container {