Update to charm-tools >= 2.0.0

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: I10271bfa64674a4789d1cc623a2a7250a154ec43

test-requirements.txt

@@ -5,4 +5,5 @@ coverage>=3.6
 mock>=1.2
 flake8>=2.2.4,<=2.4.1
 os-testr>=0.4.1
-charm-tools
+charm-tools>=2.0.0
+requests==2.6.0

tox.ini

@@ -19,7 +19,7 @@ basepython = python2.7
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} hooks unit_tests tests
-           charm proof
+           charm-proof
 
 [testenv:venv]
 commands = {posargs}