Use new Model._meta API to prevent deprecation warnings with django 1.8

[cailloumajor]

Fixes issue #68

[landscape-bot]

Repository health decreased by 0.08% when pulling 9b34469 on cailloumajor:model-meta-api into cbdef43 on paulocheque:master.

• 2 new problems were found (including 0 errors and 2 code smells).
• 1 problem was fixed (including 0 errors and 1 code smell).

[cailloumajor]

@landscape-bot : since django 1.8, Model._meta is a public API (see https://docs.djangoproject.com/en/1.8/ref/models/meta/), although you consider it as a 'protected' member.

[paulocheque]

Amazing! Thanks!