Update Windows cross build for ruby-2.6

[larskanis]

This also adds a JRuby task which makes use of new JRuby support in rake-compiler-dock-0.7.0. I'm not sure whether this is something for build_all.

Fixes #1842 .

[flavorjones]

@larskanis Thank you! Will merge in the morning and planning to cut v1.10.0 this week.

[flavorjones]

... And I've updated build_all to use the new task. Thanks again!

[flavorjones]

@larskanis I've just cut v1.10.0.rc1, could you please make sure the windows gems look OK to you?

[larskanis]

@flavorjones Looks good to me. I tested v1.10.0.rc1 on Windows server 2016:

• binary gems on ruby-2.4-i386 and ruby-2.6-x64 running the nokogiri test suite from github
• rake compile test on github master branch on ruby-2.4-i386 and ruby-2.6-x64
• gem inst --pre --platform ruby nokogiri and run tests on ruby-2.6.0

Everthing succeeded. rake compile on ruby-2.6 emitted a lot of warnings kind of:

• warning: cast discards 'const' qualifier from pointer target type
• warning: Please include winsock2.h before windows.h
  They aren't new and I believe the latter one is a ruby issue.

[flavorjones]

Awesome, thank you for performing those tests!

Yes, the compilation warnings have been around for a while and aren't new with 2.6. I've tried squashing them but couldn't find a way to do it without introducing #pragmas all over.