Blockbase: add scroll overflow to pre tags (#4674)

blockbase/assets/ponyfill.css

@@ -21,6 +21,10 @@ img {
 	box-sizing: border-box;
 }
 
+pre {
+	overflow: scroll;
+}
+
 /**
  * Breakpoints & Media Queries
  */

blockbase/sass/base/_normalize.scss

@@ -19,4 +19,8 @@ img {
 
 * {
 	box-sizing: border-box;
+}
+
+pre {
+	overflow: scroll;	
 }