Bind to all network interfaces

This default changed from 0.0.0.0 to localhost in Rails 4.2.

Fixes #61

Procfile

@@ -1 +1 @@
-web:    bundle exec rails server -p 80 -e ${RAILS_ENV:-production}
+web:    bundle exec rails server -b 0.0.0.0 -p 80 -e ${RAILS_ENV:-production}