Prepare for 2.7.2.1 release

changelog.md

 # Changelog for [`unix` package](http://hackage.haskell.org/package/unix)
 
-## 2.7.2.1  *Sep 2016*
+## 2.7.2.1  *Nov 2016*
 
   * Don't use `readdir_r` if its deprecated.
 
+  * Add argument documentation for Env modules
+
 ## 2.7.2.0  *Apr 2016*
 
   * Bundled with GHC 8.0.1

unix.cabal

 name:           unix
-version:        2.7.2.0
+version:        2.7.2.1
 -- NOTE: Don't forget to update ./changelog.md
 license:        BSD3
 license-file:   LICENSE