last-working-dir: Use >! to overwrite $cache_file

Use ">!" to overwrite $cache_file in case noclobber is set. 
When noclobber is set, zsh will not allow the use of ">" to overwrite
the contents of a file. Instead, it displays a "file exists" error.
By using ">!" instead, we tell zsh to overwrite the file without
complaining.
Fix whitespace

That's what I get for using GitHub text editor and not checking tabs.
Merge pull request #1662 from rylwin/patch-1

last-working-dir: Use >! to overwrite $cache_file