Declare django lower bound
After in openstack/requirements
repo, all entries in project requirements.txt must have lower bound
although we don't track lower bound in global-requirements.txt.
This commit re-adds Django lower bound.
pbr!=2.1.0,>=2.0.0 # Apache-2.0
# Horizon Core Requirements
Babel!=2.4.0,>=2.3.4 # BSD
-Django<2;python_version<'3.0' # BSD
-Django<2.1;python_version>='3.0' # BSD
+Django<2,>=1.11;python_version<'3.0' # BSD
+Django<2.1,>=1.11;python_version>='3.0' # BSD
django-babel>=0.6.2 # BSD
django-compressor>=2.0 # MIT
django-pyscss>=2.0.2 # BSD License (2 clause)
