Fix django-ldapdb upper version limit in requirements.txt

requirements.txt

@@ -10,7 +10,7 @@ django-polymorphic==0.7.2
 django-sendfile==0.3.10
 django-localflavor==1.1
 django-netfields>=0.4,<0.5
-django-ldapdb>=0.4.0,<5.0
+django-ldapdb>=0.4.0,<0.5.0
 django-multiselectfield>=0.1.5
 feedparser
 six==1.10.0