Updates CHANGELOG for 1.6.0 release

- Adds date for 1.6.0
- Removes empty 1.5.1 stub

Author: weierophinney

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 All notable changes to this project will be documented in this file, in reverse chronological order by release.
 
-## 1.6.0 - TBD
+## 1.6.0 - 2017-09-13
 
 ### Added
 
@@ -40,28 +40,6 @@ All notable changes to this project will be documented in this file, in reverse
 
 - Nothing.
 
-## 1.5.1 - TBD
-
-### Added
-
-- Nothing.
-
-### Changed
-
-- Nothing.
-
-### Deprecated
-
-- Nothing.
-
-### Removed
-
-- Nothing.
-
-### Fixed
-
-- Nothing.
-
 ## 1.5.0 - 2017-08-22
 
 ### Added