Problem: GHC-8.8 is a thing, we want to make sure that
we can build o-clock using stack and latest GHC.

Solution: add stack-8.8.1.yaml which uses nightly resolver
with GHC-8.8.1. Add such build to Travis config. Also add
cabal build with 8.8.1 and update o-clock.cabal (tested-with).

Problem: there are new versions of some dependencies but out
upper bounds do not permit using them.

Solution: update upper bounds to permit latest versions. I've
checked that there are apparently no breaking changes that matter
for us.

Problem: our benchmarks depend on `time-units` and `tiempo` which are
rather dead libraries. Because of that we have to specify `allow-newer`
in Stack configurations and nightly resolver does not have `o-clock`
because it cares about benchmarks and these dependencies are not in
the resolver.

Solution: so we conditionally make benchmarks not buildable. One still
can build them by passing a new flag.

There are important changes so we would like to release a new
version.