Fix bug 816

This pins the installed version of Juju to 2.5, which matches the pinned
version of libjuju used by N2VC. The mismatch is causing issues
deploying charms with new installs.

Change-Id: Ia831b7735511172a64e6e691ce87848dc5b95ae4
Signed-off-by: Adam Israel <adam.israel@canonical.com>