Make sure the symlink to the running version is created after the file download

Hey!  Great work on this module, wasn't happy with the ones on the forge already so I had been thinking of writing one, but this one's functionality is exactly what I want.

Ran into one issue during setup, the `${install_path}/serf-${version}` file was getting in place but `${install_path}/serf` was getting created as an empty file instead of a link - a second run was needed to converge and fix the link.  This change adds `ensure => link` and `require => File["serf-${version}"],` to the symlink resource, which resolve the issue.

Thanks!