Pin Ansible to <2.4

Ansible 2.4 is scheduled for release in september [1] and changes a lot
of things we might not be ready for.
Let's just make sure we control when we want to upgrade to 2.4.

The latest release of ARA, 0.14.2, does not currently support Ansible 2.4.
The support for Ansible 2.4 will likely come in 1.0.

[1]: https://github.com/ansible/ansible/blob/b3f2d1befe509614febc28d1bf1734cb2cc50ac0/docs/docsite/rst/roadmap/ROADMAP_2_4.rst

Change-Id: Icd9b847d4e75c509aba0c3a060bb6eedfd9be257
diff --git a/requirements.txt b/requirements.txt
index 69509d0..eadb508 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -16,7 +16,7 @@
 apscheduler>=3.0
 PrettyTable>=0.6,<0.8
 babel>=1.0
-ansible>=2.3.0.0
+ansible>=2.3.0.0,<2.4
 kazoo
 sqlalchemy
 alembic