AppVeyour was configured to use the latest versions of Node for major releases. To make builds more reproducible I've changed both Travis and AppVeyor to use the same fixed versions of Node 6 and Node 7 that AppVeyor is using at the moment.
This patch:
- migrates CI to use NPM
- drops lockfiles (`yarn.lock`). Lockfiles are ignored by package
managers when the package is installed as a dependency, so this makes CI closer to the
installation our clients run.
This patch:
- starts using '&&' instead of '&' so that any failed commands results
in build failure
- removes test-node6-transformer step from the node6 target
