Update Test Dependencies and create separate test environment

[alecloudenback]

In the old build environment, docs weren't generating .

I thought I would simplify the CI suite by just moving the tests and docs to GitHub Actions (should have bundled that with this PR...) and removing the Travis and Appveyor integrations (it seems like Appveyor would still need to be disabled in the repo settings, as it's still running).

Tests were failing because ExcelFiles had a Python dependency that was broken.

So in response, I:

• switch dependency from ExcelFiles to XLSX.
• Also made separate test environment to keep primary Project dependencies light

Implications/next steps

• Would make min Julia compatibility 1.2
• Would tag a v0.3.0 release due to change in dependencies

[alecloudenback]

@EricForgy / @simonbyrne any concerns, particularly with the Julia 1.2 floor or CI updates? There would always be v0.2.0 of DayCounts available for those on 1.2

[EricForgy]

It's all your's @alecloudenback 👍 No concerns from me 😊