Retain order of INSTALLED_APPS in app_paths

[fritz-k]

Commit 433b132f2402cf6ceae5b1a8ca6956933ba7798c messes up the displayed order of apps by using an unordered dict to store app_paths.

This fix retains the order of apps but leads to the order of apps being reversed. It's not bad if using a scheme like

INSTALLED_APPS = (
# django apps
...
# 3rd party apps
...
# project apps
...
)

since your most used apps are located at the top. Still this behavior might be undesirable.

[fritz-k]

Any thoughts on this? The random order of apps is kind of annoying...