Bump errortrace dependency

The improvement on the source location information of errors in
8cb765d technically requires errortrace-lib at least 1.6. See also:
racket/errotrace@39f554b4 and racket/errortrace#38.

Author: shhyou

htdp-lib/info.rkt

@@ -7,7 +7,7 @@
     "compatibility-lib"
     "draw-lib"
     ("drracket-plugin-lib" #:version "1.1")
-    "errortrace-lib"
+    ("errortrace-lib" #:version "1.6")
     "html-lib"
     "images-gui-lib"
     "images-lib"