Uninstall curl before running debian-bootstrap

debian-bootstrap attempts to install cabal-install-1.16, which can
either work with curl, wget, or fetch, in that order of preference.
Unfortunately the hackage URLs have changed since 1.16 and with the
arguments used in cabal-install's bootstrap script, curl doesn't
follow 301 redirects.

The simplest solution is to uninstall curl, so that wget is used
instead.
Upgrade to cabal-install-1.18 instead

Instead of removing curl to work around limitations in the
cabal-install-1.16 bootstrap script, we upgrade to cabal-install-1.18
instead.
Merge pull request #294 from gelisam/no-curl

Uninstall curl before running debian-bootstrap