Fix cassava doubledash

[nh2]

When trying to install keter with wget -O - https://raw.githubusercontent.com/snoyberg/keter/master/setup-keter.sh | bash I got:

...
+ stack setup
Writing implicit global project config file to: /root/.stack/global-project/stack.yaml
Note: You can change the snapshot via the resolver field there.
Using latest snapshot resolver: lts-10.3
Downloaded lts-10.3 build plan.    
AesonException "Error in $.packages.cassava.constraints.flags['bytestring--lt-0_10_4']: Invalid flag name: \"bytestring--lt-0_10_4\""

That is this issue: haskell-hvr/cassava#155

This PR fixes it by making the setup-keter.sh install script install the latest version of stack.

It also fixes the problem that the stack install used inside didn't specify the resolver, so it might use a newer ghc than keter is compatible with. In my case, it didn't build with ghc 8.2, failing with:

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for keter-1.4.3.2:
    unix-compat-0.5.0.1 from stack configuration does not match >=0.3 && <0.5 (latest
                        matching version is 0.4.3.1)
needed since keter is a build target.

So I pass --resolver giving the same resolver as in stack.yaml in this git repo.