Deps: upgrade Unicorn to 6.0.0 (stringer-rss#563)

There doesn't appear to be anything in [the major release][release] that
should impact us.

[release]: https://github.com/defunkt/unicorn/releases/tag/v6.0.0

Author: mockdeep

Gemfile.lock

@@ -195,7 +195,7 @@ GEM
     uglifier (4.2.0)
       execjs (>= 0.3.0, < 3)
     unicode-display_width (2.0.0)
-    unicorn (5.8.0)
+    unicorn (6.0.0)
       kgio (~> 2.6)
       raindrops (~> 0.7)
     will_paginate (3.3.0)