Weaken dependency on process.

See #1763

GHC 7.6 still distributes by default with an old version of process, which
means that requiring the most recent version prevents GHC from properly
bootstrapping out of the box.  In this patch, we relax the version requirement
on process, disabling proper ctl-c handling when the version is not sufficiently
new.

Perhaps one problem with lowering the bound, is if someone attempts to install
Cabal out of the box on GHC 7.6, the dependency solver will prefer the old
installed version, so it is very easy to end up with a cabal-install which
doesn't have working ctl-c.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>