Update for rspec 3.0 and correct timestamp for static files

Separating commits for bug fixes and configuration option, as per https://github.com/jekyll/jekyll-sitemap/pull/21#issuecomment-45813773

1. Update the specs to work on rspec 3.0: be_true is now be_truthy
2. Correctly format timestamps of static files: Use %S for seconds of the minute instead of %s for seconds since the UNIX epoch