add node 12 to build; remove node 11

[boneskull]

node 11 is no longer maintained

[coveralls]

Coverage remained the same at 92.353% when pulling 81a0433 on boneskull/node-12-build into f3ed729 on master.

[boneskull]

to be clear about the why's of that hack around browser tests:

1. the browser tests are the only place where we allow native modules to be downloaded and/or compiled. we need these to launch the browser tests.
2. unfortunately, this also means we get canvas, which is a transient dependency of builder.
3. canvas provides a prebuilt binary for node 10
4. canvas does not provide a prebuilt binary for node 12, which means we must compile it from source
5. ...and the prerequisites to do so are not available in the default travis environment

So we could either download all of those dependencies (which is a handful of stuff grabbed via apt) and build canvas from source which we won't even use, because it's for building the docs.... or we just run the browser tests in Node.js v10. the Node.js version doesn't really matter, because these are browser tests, after all.

[boneskull]

I would not consider this a breaking change as long as we support versions of Node.js prior to v11. We can consider this "implicit support".

in the far future, when we drop Node.js v10 support, we will be essentially dropping Node.js v11 support as well.

[boneskull]

closing in lieu of #3919