Force UTF-8 encoding on file contents

`Base64.decode64` returns string with encoding of `ASCII-8BIT`. Some linters
will try to write the file. If a file contains weird characters an error will
get raised, like:

```
 Encoding::UndefinedConversionError: "\xE2" from ASCII-8BIT to UTF-8
```

Related error:
https://app.getsentry.com/app20126465herokucom/production/group/33706905/