Base64 should be added to gemspec

Ruby 3.3.0 gives the following warning:

```
vault-0.18.2/lib/vault/defaults.rb:5: warning: base64 was loaded from the standard library
but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile
or gemspec. Also contact author of vault-0.18.2 to add base64 into its gemspec.
```