Rebuild to use charm-tools pre 2.8 release

Reverting to an earlier version of charm-tools to
resolve some building issues seen with latest 2.8.2
version.

Change-Id: Iafcbe173d9b1387b8e65cf7c8da39f0da753bdbc

requirements.txt

@@ -9,7 +9,7 @@
 setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb244e9b27bc56750c44b7c85
 
 # Build requirements
-charm-tools>=2.4.4
+charm-tools>=2.4.4,<2.8
 
 # Workaround until https://github.com/juju/charm-tools/pull/589 gets
 # published