Force padding top bottom to 1px for div.output_area pre (jupyter#3194)

See jupyter#3193

Author: Naereen

notebook/static/notebook/less/outputarea.less

@@ -95,7 +95,7 @@ div.output_area {
 
 div.output_area pre {
     margin: 0;
-    padding: 0;
+    padding: 1px 0 1px 0;
     border: 0;
     vertical-align: baseline;
     color: @output_pre_color;