Fix RFC5424 timestamp output

[daph]

This fixes issue #3, "RFC5424 output seems to format zone offset incorrectly"

Adding the second 'Z' to the timestamp format outputs the offset with the colon (see date format docs here). I tested this with syslog-ng as the receiving syslog and it now happily accepts RFC5424 messages sent to it via logstash-output-syslog where before it complained and wouldn't parse them.

[suyograo]

@daph can you rebase please?

[daph]

@suyograo Rebase done.

[suyograo]

@daph can you sign the CLA? https://github.com/elasticsearch/logstash/blob/master/CONTRIBUTING.md#contribution-steps

[daph]

@suyograo I could have sworn I already did, but oh well. I just went and signed it all now.

[suyograo]

@daph make sure to sign the CLA with the same email as the one you use for commit. I see a skytap email here, but I think you signed with a different one.

[suyograo]

@daph if it helps, you can also create a brand new PR with the new email.

[daph]

@suyograo Woops, sorry about that. Didn't realize that's the email I have for the commit. Is it fine if I just resign the CLA again, but with the skytap email?

Edit: I just went ahead and signed again with the skytap email. I'll probably have commits under that email in the future anyway.

[suyograo]

CLA check ok. LGTM. manually merging

[suyograo]

@daph thanks for your patience and contribution to Logstash!