Ensure unit tests pass with any Node.js ABI version

[lovell]

Fixes #113

• Pins to ABI 64 as this is supported by the a-native-module dependency
• Adds Node.js 12 to CI environment
• Node.js 14 is not yet supported by the verify-travis-appveyor dependency

[vweevers]

We can remove verify-travis-appveyor. In the 2 years that we used it (here and elsewhere), it never barked. In other words, we didn't make the tiny mistakes that this tool intends to prevent.

[lovell]

Thanks, have updated to remove verify-travis-appveyor and add Node.js 14 to the CI matrix.

[vweevers]

Retried node 14 on AppVeyor a bunch of times, keeps failing with exit code 1073741819 on either npm install or npm test:

npm test
Command exited with code -1073741819

[lovell]

Appveyor's x86 Node.js 14 environment has been failing about 50% of the time for other repos over the last couple of days so I don't think it is related.

[vweevers]

Link dump:

appveyor/ci#3407 (same issue but on an earlier 14.x)
https://github.com/FeodorFitsner/node-14-x86-issue (repro)
npm/cli#1227 (npm report)
nodejs/node#32863 (possibly related)