Upgrade to Django 1.8

Starts using version interval instead of version pinning, that way, patch
versions (security updates…), which do not break API will get installed without
requirements.txt modification.

requirements.txt

@@ -1,4 +1,4 @@
-Django==1.7.11
+Django>=1.8.17,<1.9
 psycopg2==2.5.2
 python-ldap==2.4.15
 wsgiref==0.1.2