Add the atlas folder to LIBRARY_PATH

To make the build pack work and able to unpickle sklearn models built offline I add to:

```
$ heroku config:set BUILDPACK_URL=https://github.com/dbrgn/heroku-buildpack-python-sklearn/
$ heroku config:add LD_LIBRARY_PATH=/app/.heroku/vendor/lib/atlas-base/atlas:/app/.heroku/vendor/lib/atlas-base:/app/.heroku/vendor/lib/
```

otherwise I get a:

```
ImportError: liblapack.so.3gf: cannot open shared object file: No such file or directory
```

when the application loads scipy.

Presumably setting LD_LIBRARY_PATH directly in the build back will remove that manual configuration step.