The old version is still around, because hledger-vty won't accept the
latest one. Note that previously the vty was build with 'mtl' overridden
to 'mtl2'. I dropped that override, because it seems to have become be
unnecessary. If anyone is aware of a hidden reason why that mtl2 must be
set explicitly, please let me know.
svn path=/nixpkgs/trunk/; revision=28258
7892062e3c