Use `get_comment_date` instead of `comment_date`

Because `comment_date` already displays / echo's it value.  
http://codex.wordpress.org/Function_Reference/comment_date  
http://codex.wordpress.org/Function_Reference/get_comment_date
Merge pull request #1065 from jmslbam/patch-1

Use `get_comment_date` instead of `comment_date`