Ignore `nodes.local.yaml`

Commit 530ff06 switched `vagrant-govuk` to use YAML for its
configuration and 7638570 updated the documentation to use
`nodes.local.yaml` rather than `nodes.local.json` for local node
configuration.

Update the `.gitignore` file to reflect the changes in the above
commits.
Merge pull request #33 from alphagov/ignore-local-yaml-config

Ignore `nodes.local.yaml`