Bump minor version to 0.5.1.0

This way the type-role addition in
4baaf0b6 becomes be detectable via
`MIN_VERSION_array()`

array.cabal

 name:          array
-version:       0.5.0.1
+version:       0.5.1.0
 -- NOTE: Don't forget to update ./changelog.md
 license:       BSD3
 license-file:  LICENSE

changelog.md

 # Changelog for [`array` package](http://hackage.haskell.org/package/array)
 
-## 0.5.0.1  *TBA*
+## 0.5.1.0  *Mar 2015*
 
   * Bundled with GHC 7.10.1
+  * Add role annotations for GHC >= 7.8 (#9220)
 
 ## 0.5.0.0  *Nov 2013*