App’s PATH not overwritten with build env PATH

The PATH generated by the buildpack previously overwrote the system’s
existing PATH.  The new behavior is to prepend the desired additions to
that existing PATH.

Including the build environment's $PATH in the export is not desirable,
as the result includes directories which will not exist on release:

    export PATH="$HOME/.jdk/bin:/app/.jdk/bin:/tmp/build_8c0d5152-61a4-4e72-8bae-0574f014ba4f/.jdk//bin::/tmp/codon/vendor/bin:/usr/ruby1.9.2/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:$HOME/.lein/bin"

Better to explicitly declare only the custom paths.

This is the same methodology used by other buildpacks:

* https://github.com/heroku/heroku-buildpack-scala/blob/37723c214e860d9cfab46a75cb9ad54bc82fbcb8/bin/compile#L127
* https://github.com/heroku/heroku-buildpack-python/blob/7ec8f126488fbadfad58ebee95b3a4443f3d238e/bin/compile#L245
* https://github.com/heroku/heroku-buildpack-nodejs/blob/a8b31de496ce56d7c8218efa4a680d42df23beeb/bin/compile#L137
Merge branch 'path-export'