Add support for Linux to the python task (Pyenv)

[pior]

The python task installs pyenv with Homebrew if it's not already installed.

This will fail on linux.

[mlhamel]

As discussed IRL:

Let's:

1. on macOS: install pyenv with homebrew.
2. on linux: fail and send a message to tell the user how we would suggest they should install pyenv.

[merwok]

What about looking for the Pythons installed by the OS? I never need to use pyenv.

[mlhamel]

If you're add a specific requirement for the python version, something like:

up:
  - python: 3.6.5

We could hope the os would give us this specific version but there's close to chance it would work

[pior]

So I guess this suggestion is linked to https://github.com/devbuddy/devbuddy/issues/199, which says that we should not over-specify the python version.

We should discuss this. We need to consider:

• how to make DevBuddy super useful on Linux
• keep compatibility with Shopify Dev

[merwok]

Counter-point to my idea of using system Python: debian/ubuntu Python maintainer backport patches, so «Python X.Y.Z» is not an exact spec, and local behaviour may not match CI or server environment. Using pyenv/pythonz is needed for reproducibility.

【For context, I am finally testing bud on a personal project; makefiles are awful, and a collection of shell scripts isn’t great either (lots of copying + shell code is bad + doesn’t work for people with Mac). To start quickly I wanted to avoid pyenv, as I am quite happy with system python and virtualenvwrapper, but it is not proving easy.】