use build stages for Travis CI

[outsideris]

Description of the Change

Introduce TravisCI build stages; it's still beta to make our builds more obviously and easier to understand. It groups our builds as lint or test.
I run linting first and then run tests because linting is much cheaper than tests.

Alternate Designs

We can add more stages.
For example, we can use a build stage instead before_script or a coverage stage instead after_success.

Why should this be in core?

IMO, it's more obviously, but it's tiny.
I'm not sure that we should use this yet. It's still beta.

Benefits

Easy to understand our build status.

Possible Drawbacks

If there are some bugs in this feature or Travis change something, our build might be broken, or we should update our .travis.yml.

Applicable issues

#3292

[coveralls]

Coverage increased (+0.05%) to 90.0% when pulling daa7c93 on outsideris:issue-3292 into 9ba8eca on mochajs:master.

[outsideris]

Coverage change is annoying. I will dig the problem when I have enough time.

[boneskull]

thanks a ton. i'm going to merge this because I want to fiddle with it.

[boneskull]

I am guessing this can be simplified into something like

script: npm start $TARGET
node_js: '9'
jobs:
   include:
     - stage: lint
        env: TARGET=lint
     - stage: test # use default node version
        env: TARGET=test.node COVERAGE=true
     - stage: test
        node_js: '8'
        env: TARGET=test.node 
     - stage: test
        node_js : '6'
        env: TARGET=test.node

see https://docs.travis-ci.com/user/build-stages/#Build-Stages-and-Build-Matrix-Expansion