Fix "warning: assigned but unused variable - base64"
Fix "warning: assigned but unused variable - base64"

This fixes following warning:

```
vendor/bundle/ruby/2.4.0/gems/loofah-2.1.0/lib/loofah/html5/scrub.rb:47: warning: assigned but unused variable - base64
```

flavorjones/loofah,flavorjones/loofah,flavorjones/loofah