add @heroku/languages as code owners

Switch nodejs install over to using nodebin

Error on yarn install if the lockfile is out of date

Error on conflict between npm shrinkwrap and modern lockfiles

Update changelog

Add $STACK to the cache signature

This reduces the wall-clock time from 16 minutes to 9 minutes.

Previously new apps (or apps where the cache has been cleared using
the heroku-repo plugin) would give a skipping cache restore message
of "new-signature" which was slightly misleading.

To ensure that the cache is busted when apps update to a new stack.

Fixes #445.

If the user has not specified a version for npm, but there is a package-lock.json we should automatically install npm 5

Fixes #422

Error out when two different lockfiles are present in the repo

Fixes #410

Update changelog

- Log every time the compile succeeds
- Add log information about which build scripts are being used

The buildpack should bust the cache whenever the PREBUILD boolean changes
to prevent the user from caching their local node_modules directory

Fixes #399

If a user pushes a source tree that already includes .heroku/node, give
a nice warning instead of a crypic error message.

Fixes #371

* Make yarn available to application if defined in engines

Fixes #402 

- Drop testing support for Cedar-10
- Add testing support for Heroku-16

Fix link to shunit2