(maint) Prefer pson over json for logstash_event

We can't always rely on json being present, since that dependency may be
satisfied by rubygems and this code isn't responsible for loading
rubygems. However we can rely on pson always being available, which
satisfies this same need.  This commits switches the logstash_event log
destination from trying to require and use json to simply use pson.
Merge pull request #1963 from adrienthebo/maint/logstash_event-json_require

(maint) Prefer json over pson for logstash_event