update maintainers list in cabal file

Author: andrewthad

unordered-containers.cabal

@@ -11,7 +11,7 @@ description:
 license:        BSD3
 license-file:   LICENSE
 author:         Johan Tibell
-maintainer:     [email protected]
+maintainer:     [email protected], [email protected]
 Homepage:       https://github.com/tibbe/unordered-containers
 bug-reports:    https://github.com/tibbe/unordered-containers/issues
 copyright:      2010-2014 Johan Tibell