Support NODE_MODULE_CACHE=false

[MrOrz]

The buildpack should provide an option to turn off the default behavior of caching node_modules.

In Node.JS buildpack, it can be achieved by setting NODE_MODULE_CACHE in the config.
https://devcenter.heroku.com/articles/nodejs-support#cache-behavior

[MrOrz]

Hmm it turns out that there is an API tool to purge Heroku caches.
https://github.com/heroku/heroku-repo#purge_cache

Supporting NODE_MODULE_CACHE seems not so urgent now.