Previously, the `integration/` tests were failing, because `concurrently "foo"`
does not inherit the `PATH` env var ([more info][1]).
This commit fixes it, by setting the `PATH` env var explicitly:
`concurrently "PATH=$PATH foo"`.
This commit also includes some minor refactoring of the `integration/` tests scripts:
- Move build-related operations to `ci-lite/build.sh` (for consistency).
- Use `yarn run ...` instead of `npm run ...` inside package.json scripts.
- Use global `yarn` (since we are already using it for `aio/`).
- Fix some `travis_fold` statements.
[1]: https://github.com/kimmobrunfeldt/concurrently/issues/61#issuecomment-252081610
