`libtinfo.so.6` is missing on Ubuntu 18.04
When I try to setup GHC with ghcup on Ubuntu 18.04 LTS, `ghc-pkg` fails to run because `libtinfo.so.6` is missing.
Using deb9 bindist seems to work.
According to https://packages.ubuntu.com/cosmic/libtinfo6, the package `libtinfo6`, which provides `libtinfo.so.6`, is only available since Ubuntu 18.10 (cosmic).
Therefore, `.download-urls` should be changed like this:
```
ghc 8.6.5 x86_64 debian=9,debian,mint,ubuntu=16.04,ubuntu=18.04 https://downloads.haskell.org/~ghc/8.6.5/ghc-8.6.5-x86_64-deb9-linux.tar.xz
```
Related issue: #108
issue