Fix typo in setup.py to make tarsnapper installable again.

Thanks for writing tarsnapper. I have been using the http://community.opscode.com/cookbooks/tarsnap cookbook to set up tarsnap on new machines, and the installation recipe for tarsnapper fails with:

"error in tarsnapper setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers"

I've tested this change and it fixes the problem.