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Error on yarn install if the lockfile is out of date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

* instrument basic metrics
* remove extraneous failure count
* better warning and failure hierarchy
* remove strict unset check to be compatible with stdlib

This reverts commit dd6a5f5a.

Warnings that mention npm installs can be confusing if you're using yarn to install your dependencies.