avoid error when there are no untracked files

check whether there are any untracked files before deleting them

````
remote: | private method `readlines' called for "":String
remote: | /usr/local/bin/puppet-sync:117:in `block in update'
remote: | /usr/local/bin/puppet-sync:104:in `chdir'
remote: | /usr/local/bin/puppet-sync:104:in `update'
remote: | /usr/local/bin/puppet-sync:79:in `run'
remote: | /usr/local/bin/puppet-sync:221:in `<main>'
````
Merge pull request #2 from dhgwilliam/master

check output of git(:ls-files) before iterating
Fix spelling error in example post-receive filename
Update README file with current usage