- strings are now assumed to always be Unicode
This fixes a bug where this caused a crash:

LANG=C bin/logstash agent -e 'filter { mutate { replace => { "message"
=> "☹" } } } output { stdout { codec => json } }'

The problem was that LANG=C told ruby's eval() what the encoding of
strings was by default, and since logstash requires UTF-8/unicode
elswhere, it makes sense to require it for the config file as well.