Fix and improve S3/Heroku integration
Fix and improve S3/Heroku integration

- The existing code caused exceptions in all environments but production. This is now fixed.
- No need to use a constant – a regular variable will do just fine.
- Use Pathname's own #exist? method rather than FileTest.
- There should be no behavior that depends on the Rails environment. It is perfectly acceptable to
use Heroku-like behavior if the user decides to set the according environment variables or use the
config file if the user decides that this makes sense.

daemonsy/spree_showcase,mileer/spree_showcase,daemonsy/spree_showcase,mileer/spree_showcase,daemonsy/spree_showcase