Improve local testability

Running Pkg.test("MLJFlow") locally requires that an MLflow service is already running on your machine and that the uri "http://localhost:5000" will work to connect to it. On my mac, that uri will not work and I must manually edit it to be "http://127.0.1:5000", which is a pain.

Here's one suggestion: To run tests one must set a local env variable "MLFLOW_URI" to the uri of an active MLflow service. If the env is empty a helpful warning explaining what to do is thrown.

@pebeto Do you have any other suggestions?

In this case, are we going to set a default value for MLFLOW_URI?

I consider this change like something practical, but it can be tedious for the end-user. However, not all of us have mljflow running on the same port, so I think it's good to go.

| In this case, are we going to set a default value for MLFLOW_URI?

The problem with setting a default is this: If we get an error, how do we know it's because the URI's don't match and not for some other reason?

Reopening as not complete. We need to:

Change the URI's in tests to the one set by the environment var. There at two places, here is one:

MLJFlow.jl/test/types.jl

Line 2 in 4387aef

logger = MLJFlow.Logger("http://localhost:5000")
Set the env variable appropriately in CI.yml

@pebeto In the future, can we please have reviewable PR's for new changes, rather than direct commits to dev?