Update changelog

Author: Daniel O'Connor

CHANGELOG.md

@@ -4,7 +4,7 @@ It does not matter how slowly you go as long as you do not stop.
 First they ignore you, then they laugh at you, then they fight you,
 then you win. –- Mahatma Gandhi
 
-Unreleased (0.15.0)
+Thu Dec 14, 2017 (0.15.0)
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 This release upgrades to rails 5.0.0
  - #500 - Upgrade rails