Set up Mac build on SemaphoreCI v2

[bahmutov]

To confirm cypress-io/cypress#918 (comment) issue

_RegisterApplication(), FAILED TO establish the default connection to the WindowServer, _CGSDefaultConnection() is NULL.
2019-06-26 05:07:07.441 Cypress[5288:16143] +[NSXPCSharedListener endpointForReply:withListenerName:]: an error occurred while attempting to obtain endpoint for listener 'ClientCallsAuxiliary': Connection interrupted

====================================================================================================

  (Run Starting)

  ┌────────────────────────────────────────────────────────────────────────────────────────────────┐
  │ Cypress:    3.3.1                                                                              │
  │ Browser:    Electron 61 (headless)                                                             │
  │ Specs:      1 found (swithing_views_spec.js)                                       │
  └────────────────────────────────────────────────────────────────────────────────────────────────┘


────────────────────────────────────────────────────────────────────────────────────────────────────

  Running: swithing_views_spec.js...                                           (1 of 1)
Cannot create BrowserWindow before app is ready
Error: Cannot create BrowserWindow before app is ready
    at Object._newBrowserWindow (/Users/semaphore/Library/Caches/Cypress/3.3.1/Cypress.app/Contents/Resources/app/packages/server/lib/gui/windows.js:104:14)
    at Object.create (/Users/semaphore/Library/Caches/Cypress/3.3.1/Cypress.app/Contents/Resources/app/packages/server/lib/gui/windows.js:207:18)
    at Object._render (/Users/semaphore/Library/Caches/Cypress/3.3.1/Cypress.app/Contents/Resources/app/packages/server/lib/browsers/electron.js:87:21)
    at /Users/semaphore/Library/Caches/Cypress/3.3.1/Cypress.app/Contents/Resources/app/packages/server/lib/browsers/electron.js:214:24
    at <anonymous>:null:null

[bahmutov]

Perfect, I can recreate the problem, see https://cypress-io.semaphoreci.com/jobs/73db74af-dbc3-4b2a-836c-42d59f3bf353

I have disabled the Mac block for now, but can be re-enabled to test, see https://github.com/cypress-io/cypress-example-kitchensink/blob/master/.semaphore/semaphore.yml#L14