Makefile does not define tox lint tests correctly

When I run bundletester, I consistently get an error about flake8 not being found. I believe that this is because:

1) I evilly do not have flake8 installed in my global Python environment.
2) The Makefile in this layer incorrectly added the py34 and py35 tox virtual environments to its path.

Instead, the Makefile should add the "lint" virtual environment to its path, as below:

```
.PHONY: lint
lint: apt_prereqs
@tox --notest
@PATH=.tox/lint/bin flake8 $(wildcard hooks reactive lib unit_tests tests)
@charm proof
```

I am planning on submitting a patch for this soon -- wanted to record in case I get sidetracked, though.

Start in the Makefile and compare the lint target with the reported tox environments and PATH. Run bundletester to reproduce the missing flake8 error, then use the issue's proposed lint target and verify the lint command and charm proof step complete successfully.

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