(maint) travis: move slack token into rooms section
(maint) travis: move slack token into rooms section

Without this, adding a custom template would cause notifications to
fail entirely.

cf. https://github.com/travis-ci/travis-ci/issues/6930#issuecomment-307606237

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb