Fix django-ldapdb version required

requirements.txt

@@ -11,7 +11,7 @@ django-sendfile==0.3.10
 django-hijack>=2.1.10,<2.2
 django-localflavor==1.1
 django-netfields>=0.4,<0.5
-django-ldapdb>=0.4.0,<5.0
+django-ldapdb>=0.4.0,<0.5
 django-multiselectfield>=0.1.5
 feedparser
 six==1.10.0