Update default branch in Gatling machine class
Update default branch in Gatling machine class

We are updating https://github.com/alphagov/govuk-load-testing to
use `main` as the default branch.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet