Appveyor - Create fat-binary gems and test - #176

Thought Appveyor was active here. The job in my fork is at:

https://ci.appveyor.com/project/MSP-Greg/bcrypt-ruby/build/1

Some benefits of testing on Appveyor:

1. Trunk build Is fully tested. Only the most recent passing build will be used. The ruby-head commit (svn 63718) that the Travis build used for this PR actually failed on ruby/ruby.
2. Ruby versions 2.4 and forward use gcc 7.3.0, which I believe is much newer than the current compilers on Travis. Appveyor has shown issues in Ruby trunk testing that weren't caught on Travis.
3. Did I mention that this took less time to complete than the respective Travis build? You might as well build fat-binary gems while you're waiting...

The question everyone wants the answer to - what needs to be done when Ruby 2.6 is released?

The answer - Nothing. Wait for Appveyor to add it to their images. Ruby 2.5 took about a month. I suspect they'll add future versions a bit quicker in the future...

@tjschuck @tenderlove @larskanis

If any of you have the time to read it and reply, I've added an issue at the repo containing the shared code for the topic of Windows pre-compiled gems and testing on Appveyor.

MSP-Greg/av-gem-build-test#1

Thanks, Greg

Per #173 and #174, I think the correct way forward is to drop the precompiled binaries altogether. (For what it's worth, this would also be a step back in our fat-binary support in the bcrypt gem — I currently precompile binaries for Windows all the way back to 1.8.7 via https://github.com/tjschuck/rake-compiler-dev-box.)

Thanks for your work on this!