fix HHVM builds failing due to non-existant directory

disable newrelic earlier during builds to prevent pushes from hanging…

… in certain rare cases

refactor newrelic and zend-opcache install routines

HHVM/3.2.0

PHP/5.5.15

fix bogus error on empty composer.json

clean up and simplify some error handling

parse optional '.git' in BUILDPACK_URL when determining branch

export only uppercase config vars, and add HOME, BUILDPACK_URL and ST…

…ACK to the blacklist

force newrelic to log to stderr; stdout interferes with boots