Fix and update setup of tests

[tdeekens]

This pull request tries to fix the setup of the tests.

Background & Context

1. Adds BABEL_ENV to test:ci
2. Adds cross-env to test
3. Moves testing on BrowserStack to node@6

Please try to run BS tests locally by e.g. tweaking the npm run-script in the package.json to

- "test:ci": "cross-env NODE_ENV=test BABEL_ENV=rollup npm run lint && npm run test:jsdom && (([ \"${TRAVIS_PULL_REQUEST}\" != \"false\" ] || [ \"${TEST_BROWSERSTACK}\" != \"true\" ]) || karma start test/karma.conf.js --log-level error --reporters dots --single-run)",
+ "test:ci": "cross-env NODE_ENV=test BABEL_ENV=rollup BS_USERNAME=xxx BS_ACCESSKEY=yyy karma start test/karma.conf.js --log-level error --reporters dots --single-run)",

[tdeekens]

I've got trouble reproducing tests failing for the same reasons as after merging #206.

1. Locally against Chrome passes and the non transpiled require which throws on BS is transpiled when debugging the launched instance
2. Running the tests against BS also works without getting the exception from the require

Ok, you merged while I was typing this 🗡

[cure53]

Oops, it sounded doneish :D

[tdeekens]

Sorry for not being clear enough :). Can you try run against BS locally to reproduce the error? I fail so far.

[cure53]

Locally things work