Fix input group sizing in Firefox

Firefox requires a width and flex-basis value to size the input field correctly. Despite the width being 1%, the input will size correctly at all parent widths.

http://codepen.io/zalog/pen/bpMJmv

scss/_input-group.scss

@@ -26,7 +26,10 @@
     }
 
     @if $enable-flex {
-      flex: 1;
+      flex: 1 1 auto;
+      // Add width 1% and flex-basis auto to ensure that button will not wrap out
+      // the column. Applies to IE Edge+ and Firefox. Chrome do not require this.
+      width: 1%;
     } @else {
       // IE9 fubars the placeholder attribute in text inputs and the arrows on
       // select elements in input groups. To fix it, we float the input. Details: