Add BUILD_DIR to env var blacklist
Add BUILD_DIR to env var blacklist

Scalingo/nodejs-buildpack,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,apiaryio/heroku-buildpack-nodejs-grunt,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,heroku/heroku-buildpack-nodejs,Scalingo/nodejs-buildpack,heroku/heroku-buildpack-nodejs