remove to_epoc

to_epoc converts "event.timestamp" to a unix timestamp, but
if the argument is not a Time object it firsts tries Time.parse(arg)

However, event.timestamp should always return a LogStash::Timestamp
object, so the method will generate an exception since passing
a Logstash::Timestamp to Time.parse coerces it to a string that
Time.parse can't handle.

Fixes #3