Remove Appveyor, replace Travis with CircleCI

[yanokwa]

My guess is that Appveyor was used to verify Windows support, but pyxform is pretty platform agnostic, so I don't think think it's needed.

Travis works fine, but the ODK tools all use CircleCI and I think some consistency there would be nice. One tricky thing here will be running the tests on the various versions of Python.

[ukanga]

In terms of making pyxform ODK tools compliant I agree. However, where windows is concerned, we introduced AppVeyor after conversations in this PR #116 necessitated it. I deploy pyxform only on Linux environments but that is not the case for all pyxform users. Can this be reconsidered?

[yanokwa]

Seems like the core of the issue is the install (See #63 (comment)). Agreed that we should keep Appveyor, but maybe replace Travis with CircleCI? I've updated the PR so you see what that looks like.

[ukanga]

... replace Travis with CircleCI?

Ok with me.