Add "unless" highlighting

[tianon]

http://docs.puppetlabs.com/puppet/latest/reference/lang_conditional.html#unless-statements

[puppetcla]

Waiting for CLA signature by @tianon

@tianon - We require a Contributor License Agreement (CLA) for people who contribute to Puppet, but we have an easy click-through license with instructions, which is available at https://cla.puppetlabs.com/

Note: if your contribution is trivial and you think it may be exempt from the CLA, please post a short reply to this comment with details. http://docs.puppetlabs.com/community/trivial_patch_exemption.html

[tianon]

It is a very trivial patch (adding a single word to the list of highlights), but I've signed the CLA anyhow to cover any issues or delays with possible future contributions.

[puppetcla]

CLA signed by all contributors.

[ssgelm]

Any chance of this getting merged?