Add missing mkdirp dependency

[salomvary]

Without this a fresh install using Jest fails:

  ● Test suite failed to run

    Cannot find module 'mkdirp' from 'node_modules/unexpected-react/lib/helpers/snapshots.js'

    Require stack:
      node_modules/unexpected-react/lib/helpers/snapshots.js
      node_modules/unexpected-react/lib/assertions/jestSnapshotTestRendererAssertions.js
      node_modules/unexpected-react/lib/test-renderer-jest.js
      node_modules/unexpected-react/test-renderer-jest.js
      src/__tests__/ClickCounter.spec.js