Unbreak "require 'logstash-event'".

Do not require logstash/version.rb in logstash-event.rb because that is not
shipped anymore.

Also remove unused require in the logstash-event.gemspec.
Merge pull request #626 from bernd/fix-logstash-event

Unbreak "require 'logstash-event'"
- fix a duplicate 'message' field (overwrite it)
- update to new version of insist (which add support for .is_a?)
- restore -t/--configtest flag accidentally removed in the previous
released (LOGSTASH-1279)

Testing -
  % bin/logstash agent -e '' --configtest
  Configuration OK

  % bin/logstash agent -e 'asdf' --configtest
  Error: Expected one of #, input, filter, output at line 1, column 1
  (byte 1) after
Fix 'agent -- web' invocation' (LOGSTASH-1307)
Shutdown kibana/web on SIGINT
- don't print
- fix variable name typo (LOGSTASH-1296)
Merge git://github.com/logstash/logstash