Thanks but that doesn't seem to work either:

 cabal install --allow-newer=base --allow-newer=bytestring,deepseq -j3 cabal-install
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: cabal-install-1.20.0.6 (user goal)
trying: base-4.8.0.0/installed-779... (dependency of cabal-install-1.20.0.6)
next goal: process (dependency of cabal-install-1.20.0.6)
rejecting: process-1.2.1.0/installed-2db... (conflict: unix==2.7.1.0, process
=> unix==2.7.1.0/installed-4ae...)
trying: process-1.2.1.0
next goal: directory (dependency of cabal-install-1.20.0.6)
rejecting: directory-1.2.1.1/installed-b08... (conflict: directory =>
time==1.5.0.1/installed-c23..., cabal-install => time>=1.1 && <1.5)
rejecting: directory-1.2.1.0 (conflict: base==4.8.0.0/installed-779...,
directory => base>=4.5 && <4.8)
rejecting: directory-1.2.0.1, 1.2.0.0 (conflict:
base==4.8.0.0/installed-779..., directory => base>=4.2 && <4.7)
rejecting: directory-1.1.0.2 (conflict: base==4.8.0.0/installed-779...,
directory => base>=4.2 && <4.6)
rejecting: directory-1.1.0.1 (conflict: base==4.8.0.0/installed-779...,
directory => base>=4.2 && <4.5)
rejecting: directory-1.1.0.0 (conflict: base==4.8.0.0/installed-779...,
directory => base>=4.2 && <4.4)
rejecting: directory-1.0.1.2, 1.0.1.1, 1.0.1.0, 1.0.0.3, 1.0.0.0 (conflict:
process => directory>=1.1 && <1.3)
Dependency tree exhaustively searched.