Overwrite .sdirs file instead of mv'ing it

When the ~/.sdirs file is altered, bashmarks always mv's the new file to the home directory. If one is using an symlink to store the .sdirs file in another directory (for dropbox, etc.), then this symlink is blown away every time a bookmark is added/removed.

Pull request 15 (commit https://github.com/2suggestions/bashmarks/commit/05c18382ee1ab88c47a22f95b659863483377ed4) suggests using mv -f, but I feel that this solution addresses this problem, and also handles symlinks correctly.