Allow to create a self-signed cert on Ruby 1.8

`Base64.urlsafe_encode64` is only available on Ruby 1.9. This pull request ensures that if we are on Ruby 1.8, we have an equivalent implementation available.

As an alternative you could just use `Base64.encode64`, which is available in 1.8 and 1.9 and as far as I have seen it should also work here. It creates a slightly different Base64 representation (RFC 4648 with URL and Filename Safe Alphabet vs. RFC 2045) but as we don't use the generated string anywhere in filenames or URLs, we should be safe here.