Fix line status nth-child alternating colors and improve missed-branch readability

Remove invalid <div> wrapper around <li> in source_file.erb so that
nth-child(odd/even) CSS selectors work correctly for all line statuses.
Lighten missed-branch background colors for better readability when
syntax highlighting renders code as light gray text.

Closes #98.

Author: sferik

assets/stylesheets/screen.css

@@ -335,9 +335,9 @@ thead th {
 }
 
 .source_table .missed-branch:nth-child(odd) {
-  background-color: #cc8e8e;
+  background-color: #ffddbb;
 }
 
 .source_table .missed-branch:nth-child(even) {
-  background-color: #cc6e6e;
+  background-color: #ffccaa;
 }