The resolves an issue where a user that is not using a version of Node supported by the metrics plugin (not 8, 9, or 10) opts in to the feature.

Since the plugin for the appropriate version of Node is copied into the slug at build time, but the metrics url is set at runtime, we need to verify that the plugin exists before including it.

* Add metrics plugin + unit tests

This merges in the build changes and plugin from https://github.com/heroku/heroku-nodejs-metrics-buildpack which installs the metrics plugin https://github.com/heroku/heroku-nodejs-plugin at runtime once a Heroku use opts in to the language metrics feature

* Fix travis builds

- Yarn reworked the structure of their cache directory with a recent release, which broke one of the tests.

- added new fixtures for Node versions: 6, 8, 9, and 10

- Added new utilities for asserting that a file exists or does not exist after a build

Add buildpack.toml and publish script

Add heroku-18 to tests

Add more error detection for errors involving stack traces

Add detection for module 404s

Skip pruning for versions of npm with known issues

The following versions of npm will strip out dependencies installed via git when you run npm prune --production: 5.6.0, 5.5.1, 5.5.0, 5.4.2, 5.4.1, 5.4.0, 5.3.0 (didn't work for other reasons), 5.2.0, 5.1.0

This commit skips the prune step and logs a warning for these versions.

Fixes #431 

Many users getting started with Heroku try to run a build step with webpack, gulp, etc only to find that it's in their devDependencies and not installed by default on Heroku. They then have to figure out how to tell Heroku to install these. Defaulting to installing everything makes for a smoother onboarding.

Changes the default behavior to install `devDependencies`, run the build step, and then prune the `devDependencies` once the build step is complete.

Yarn versions lower than 0.19 will crash if passed --frozen-lockfile, but usage of these versions has dropped to near-zero.

Fixes #414 #504

Export cache config vars

Fix typo for grep.

Add metrics for more types of failures

Update changelog

Since the last usage of it was removed in #451.

* Move the default Yarn cache directory to a tmp dir
* Move the default npm cache directory to a tmp dir

if an app has a .profile.d script that does set -u, and that is evaluated before WEB_CONCURRENCY.sh (because of alpha order when globbing), the access to the variable, if unset, would cause a failure on startup

the fix is to check if the variable is set, using the '+' parameter substitution