update version to match tags in source repository

this was previously failing on chef runs with:
```
==> ERROR: git[/var/chef/cache/wal-e] (wal-e::default line 33) had an error: Chef::Exceptions::UnresolvableGitReference: Unable to parse SHA reference for '0.6.5' in repository 'https://github.com/wal-e/wal-e.git'. Verify your (case-sensitive) repository URL and revision.
```