Version bump to 1.6.8.0

changelog.md

 # Changelog for [`process` package](http://hackage.haskell.org/package/process)
 
-## Unreleased changes
+## 1.6.8.0 *February 2020*
 
 * Fix several bugs on Windows where use of process jobs would result
   in the process being prematurely terminated. See

process.cabal

 name:          process
-version:       1.6.7.0
+version:       1.6.8.0
 -- NOTE: Don't forget to update ./changelog.md
 license:       BSD3
 license-file:  LICENSE