Fix FolderPermission.__str__ method

Use `self.get_type_display()` instead of manually getting correct item from `self.TYPES`. This will display correctly the type label instead of something like `<django.utils.functional.__proxy__ object at 0xa9b5b4c>`
Merge pull request #465 from vstoykov/patch-1

Fix FolderPermission.__str__ method
Use the new build env on Travis

more ram, dedicated cpu cores, faster vm boot times, improved networking
fix Django migrations to support custom Image model (FILER_IMAGE_MODEL) dependency
Merge pull request #469 from joshk/patch-1

Use the new build env on Travis
Merge pull request #471 from skirsdeda/develop

fix Django migrations to support custom Image model (FILER_IMAGE_MODEL)
Document the custom image model app_meta
Merge pull request #473 from nephila/app_label

Document the custom image model app_meta