Converting project to 'stack'

[tmbull]

Hello @davnils,

I propose converting spoty to use stack. If you are unfamiliar, stack is an easy to build tool that sits on top of cabal. It aids in creating reproducible builds as it will install ghc for you in a sandbox, and allows you to specify a resolver which is a subset of commonly used hackage packages which are known to build consistently together. This is nice, because once a resolver is specified, the dependencies are "frozen" so the project should build at any time in the future. This is particularly useful for a project such a spoty which is still useful, but maybe not in active development.

[tmbull]

I have attempted to update .travis.yml to build with stack, but unfortunately, it does not seem to be using the .travis.yml from the PR for the build. This might be a security restriction on Travis CI. I'm not too experienced with Travis CI, so I might need your help testing my changes with Travis.