Don't install pandoc with apt-get. Follow those instructions to get the latest version, You need haskell-platform. Don't install haskell with apt-get, too. Use this guide
how to install haskell on ubuntu. As soon as you are done, you can install pandoc with this:
$ cabal update $ cabal install pandoc
Remember to do this NOT with sudo, because you want to have cabal locally available with the current user. But if it fails with:
Resolving dependencies... cabal: dependencies conflict: base-18.104.22.168 requires syb ==0.1.0.2 however syb-0.1.0.2 was excluded because json-0.5 requires syb >=0.3.3
you did accidently installed haskell-platform with apt-get. Do
$ sudo apt-get remove haskell-platform
and follow the link from the beginning of this post!