The new release of charm-tools no longer ships the charm
command; update minimum version requirement and switch
to using charm-proof instead, unblocking current pep8
failures across all charms.

Also pin the version of requests to 2.6.0 until theblues
(indirect dependency of charm-tools) sort out its
requirements versioning.

Change-Id: I0431192e165353aa2fd414f06afaf6ce132f9970