Load environment variables from .profile.d

During compile process, buildpacks creates a .profile.d with scripts to set up a default environment. More info at https://devcenter.heroku.com/articles/profiled.

To make this possible, we created a /etc/profile that loads app's .profile.d scripts. https://github.com/openruko/dynohost/commit/eee504878ff274352f1cb2f28d0de884fabd71d2

To automatically sources /etc/profile before executing any command, we added --login argument to bash. https://github.com/openruko/rukorun/commit/1af0b5640f40b91fde2533a37b51c8ac61a4f47e

These commits at Dynohost and rukorun are available at branch `buildpack_env`.

BTW, this PR fixes issue #22 :smile: 
All feedbacks are welcomed!