Set line-height on .button

Used same `$selectric-inner-height` variable for line-height, to center the caret icon vertically (when not using pseudo triangle).

Author: breadadams

public/selectric.css

@@ -42,6 +42,7 @@
   top: 0;
   width: 38px;
   height: 38px;
+  line-height: 38px;
   background-color: #F8f8f8;
   color: #BBB;
   text-align: center;

src/selectric.scss

@@ -46,6 +46,7 @@ $selectric-font-size:       12px !default;
     top: 0;
     width: $selectric-inner-height;
     height: $selectric-inner-height;
+    line-height: $selectric-inner-height;
     background-color: $selectric-btn-bg-color;
     color: $selectric-secondary-color;
     text-align: center;