Replace Homebrew with Debian where avaiable.

- Replace a bunch of tools installed via Homebrew with Debian packages
  from Cloudposse's repository (I find it cleaner, faster and overall
better).
- Add tfenv, remove various Terraform versions.
- Install Helm2 package instead of downloading the binaries.
- Remove the helm2 symlink from Homebrew (should have been removed
  before).