Pin juju version to 2.8.1

Change-Id: Id40b3eeb698939131faf56fe008be335e6398cef
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/setup.py b/setup.py
index 6c8d361..96593ba 100644
--- a/setup.py
+++ b/setup.py
@@ -23,7 +23,7 @@
     packages=find_packages(
         exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
     install_requires=[
-        'juju',
+        'juju==2.8.1',
         'paramiko',
         'pyasn1>=0.4.4',
     ],