Fix #398 -- Set explict GDAL and GEOS location (#478)

Set GDAL and GEOS library locaiton explicitly in environment
variables:

*   GDAL_LIBRARY_PATH "/app/.heroku/vendor/lib/libgdal.so"
*   GEOS_LIBRARY_PATH "/app/.heroku/vendor/lib/libgeos_c.so"

Django has to settings with the same name. The setup now works as
described here:
https://devcenter.heroku.com/articles/postgis#geodjango-setup