Add unix-time constraint to cabal file

[alanz]

This makes sure PR #1304 is complete, constraining the cabal build as
well, for Windows 7

[jneira]

Maybe it would be not needed: see #1304 (comment)
I am currently tested a cabal new-build on master (without bounds for unix-time) and:

Resolving dependencies...
Build profile: -w ghc-8.6.5 -O1
In order, the following will be built (use -v for more details):
 - Cabal-2.4.1.0 (lib) (requires build)
....
- unix-time-0.4.7 (lib:unix-time) (requires download & build)
....

But maybe it is safer to add it in case an upper bound is added upstream (although it would be rare)

[alanz]

Appveyor failure on one build unrelated to this. Going to merge