Always replace the bash and zsh rc files.

Sourcing the .custom(bash|zsh)rc file should allow people to customize
their configuration while allowing us to constantly maintain the files
we need in order to set up the ruby primer.
Updating the bashrc to mention that the file is managed.
Add a managed warning to the zshrc file.
Merge pull request #210 from klynton/fix-userprefs-2

Always replace the bash and zsh rc files.

I'm going to merge this and then refactor how the files are sourced.