tools: simplify stack preamble in hakyll-std.hs

This doesn't yet quite install its deps when you run it from the
makefile, you may have to run the script directly to get the deps
(hakyll, pandoc) installed.

doc/site/hakyll-std.hs

@@ -1,5 +1,5 @@
 #!/usr/bin/env stack
--- stack --resolver nightly-2015-07-01 --install-ghc runghc --package hakyll --package pandoc
+-- stack runghc --package hakyll --package pandoc
 {-# LANGUAGE OverloadedStrings #-}
 {- |