Test suite fails on Windows

[mojavelinux]

We recently discovered that the AppVeyor builds weren't running as we had thought. That means the gem was not being tested on Windows. Once we reactivated the AppVeyor builds, we discovered that the test suite fails on Windows. This issue is about getting the tests running on Windows.

Here are the problems to address:

• Calls to the asciidoctor-pdf bin script must be prefixed with ruby (since Windows can't run a shebang script and the .bat script hasn't been generated in this mode)
• Test on both Ruby 2.3 and Ruby 2.6 (2.7 not yet available on AppVeyor)
• Fix SSL verification issue on Ruby 2.3
• Define PYGMENTS_VERSION environment variable in AppVeyor config