Fix deploy error due to new pip / old setuptools conflict

Newer versions of pip (>=19.0) don't work with older versions of
setuptools (<40) due to an attribute error.  If
`include_system_packages` is `false` (now the default), this is not an
issue because of the newer setuptools that gets installed into the venv.
However, when it's `true`, which is required for some charms, the
install fails because pip somehow prefers the older system-installed
setuptools over the newer one in the venv. Pinning pip avoids the
problem until we can find a better solution.

See:
  * https://discourse.jujucharms.com/t/wheel-building-fails-during-charm-deployment/1947
  * pypa/pip#6164

wheelhouse.txt

@@ -1,4 +1,4 @@
-pip>=18.1,<19.2
+pip>=18.1,<19.0
 setuptools<42
 setuptools-scm<=1.17.0
 charmhelpers>=0.4.0,<1.0.0