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.

Empieza en el Makefile y compara el objetivo lint con los entornos tox indicados y PATH. Ejecuta bundletester para reproducir el error de flake8 faltante, luego usa el objetivo lint propuesto en el issue y verifica que el comando lint y el paso de charm proof se completen correctamente.

Escrito por el modelo de indexación a partir del texto del issue.