Set home to build_dir while sourcing

Hello,

It seems that heroku `$HOME` and `$1` ( build dir ) are not the same directory anymore during builds.

The `$HOME/vendor/bundle/ruby/2.1.0/bin` is sourced at `/app/vendor/bundle/ruby/2.1.0/bin` but the build directory is something like `/tmp/build_f95cfb33-0a6f-4891-ad78-106d4dd14467` so the bundle binary at `/tmp/build_f95cfb33-0a6f-4891-ad78-106d4dd14467/vendor/bundle/ruby/2.1.0/bin/bundle` is not in the PATH.

I made a little change to set the `$HOME` to the build dir path during the build.

Thanks for that buildpack and thanks for all the greats products from hashicorp.