Fix for bytecompiling warnings

I got following warnings at byte compiling.
I fix for no warnings.

````
Compiling file /home/syohei/tmp/gomi/coffee-mode.el at Fri Jan 25 23:57:09 2013
Entering directory `/home/syohei/tmp/gomi/'

In coffee-compiled-file-name:
coffee-mode.el:256:13:Warning: assignment to free variable `working-on-file'
coffee-mode.el:256:76:Warning: reference to free variable `working-on-file'

In coffee-propertize-function:
coffee-mode.el:835:28:Warning: `next-line' used from Lisp code
That command is designed for interactive use only
````

Please review this patch.