Add Node v12 in test matrix instead v11

[outsideris]

Today Node v12 has been released. So, it's a new current version of Node.
Test mocha with Node v12 instead of v11.

[outsideris]

AppVeyor has a problem with Node v12. I will dig it.

[juergba]

boneskull/node-environment-flags should also be revised.
The Node flags for the release v12 have to be added.

[juergba]

@outsideris I just realised it's not necessary to update "boneskull/node-environment-flags".:

If the current Node.js version is v10.10.0 or newer, the native implementation will be provided instead.

=> native implementation: process.allowedNodeEnvironmentFlags

[juergba]

Travis also fails with:

1) node-flags
       isNodeFlag()
         special cases
           should return true for flags starting with "preserve-symlinks":
     UnexpectedError: 
expected true to be false

see PR #3900

[coveralls]

Coverage increased (+0.07%) to 91.888% when pulling dc833e5 on outsideris:node-v12 into 9b00fed on mochajs:master.

[plroebuck]

Rebase and try this again.

[outsideris]

I already rebased this. However, there is a weired error at npm ci.
Also, AppVeyor looks that still doesn't to support node v12. So, I'm trying to test it with Travis Windows Supporting.

[boneskull]

I think we should go with #3919, ty.