Use venv and bundler to install dependencies

[rfc2822]

Create "prepare.sh" that:

• Sets up Python venv with explicit requirements file to install Python dependencies locally.
• Uses bundler and Gemfile to install the Ruby dependencies locally.
• Installs goat (go dependency) locally.

So when prepare.sh is run (and the necessary system packages are installed), everything is ready to run build.sh.

[rfc2822]

Done in #103