Tell users to install bundle locally without sudo.

On the current README install procedure, the command `bundle/install` fails because it tries to install gems to a shared location and that requires sudo.

There are two alternatives:

- install locally, as proposed in this MR.
- add the user to the sudo group

I chose to install locally because it:

- isolates things better
- gives less destructive powers to the `gitlab_ci_runner` user