fix: support Cypress v5.0 (test-retries)

Hi, Cypress version `5.0` is landing soon, and included in that release is [`test-retries`](https://github.com/cypress-io/cypress/pull/3968), which will break this plugin. Thankfully only a few changes were needed to support `5.0`, so I've included them in this PR. Although the changes in this PR will not break Cypress versions <5.0, This PR's tests will fail since it's expecting test-retries support. So I would recommend waiting for the Cypress `5.0` release and then updating to run against `5.0` before merging. > Note: this will also fix compatibility with `cypress-plugin-retries` (obsolete with 5.0) ## Additional Info to test against the 5.0 prerelease, I've used the following (Linux/docker): ```sh export CYPRESS_INSTALL_BINARY=https://cdn.cypress.io/beta/binary/5.0.0/linux-x64/circle-test-retries-b0e7db25b4d6c91010b09f6ff4404bf5a7bcb74b-417938/cypress.zip npm install --no-save https://cdn.cypress.io/beta/npm/5.0.0/circle-test-retries-b0e7db25b4d6c91010b09f6ff4404bf5a7bcb74b-417890/cypress.tgz npm run cy:run # or cy:open ``` ### Edit 5.0 is released, I've added CI test runs for 5.0 in addition to previous versions of Cypress.