Skip diff tests on Windows

[errordeveloper]

Description

As discussed in #449.

Checklist

• Code compiles correctly (i.e make build)
• All tests passing (i.e. make test)

[errordeveloper]

@dcherman did you have a chance to take this for a quick test?

[dcherman]

@errordeveloper I pulled down that branch and gave it a test run this morning. It definitely gets by the diff tests now, so that is good to go. It still hangs on the lint part of it, but I haven't done a lot of triage on that yet. One thing that I did note is that unless I'm mistaken, it's linting all of the vendor folder which takes a significant amount of time and is also 3rd party code.

[errordeveloper]

...it's linting all of the vendor folder which takes a significant amount of time and is also 3rd party code.

That's not supposed to happen, see linter config:

https://github.com/weaveworks/eksctl/blob/380d6fc5a8fc3ef8939e9d0a7486044ec2e6599d/.gometalinter.json#L1-L16

I'll merge this, and we can move on with build instructions.