refactor(react-modal): Stub reactTransitionGroupChild
refactor(react-modal): Stub reactTransitionGroupChild

- Animations were causing timing issues in our tests, this stubs the
  reactTransitionGroupChild so that the animation classes are applied
  immediately

[Finishes #85729922]

Signed-off-by: Nicole Sullivan <4ab7e2960369063855aad4ef791fa27066322d72@pivotal.io>

sjolicoeur/pivotal-ui,sjolicoeur/pivotal-ui,pivotal-cf/pivotal-ui,sjolicoeur/pivotal-ui,CanopyCloud/pivotal-ui,CanopyCloud/pivotal-ui,sjolicoeur/pivotal-ui,CanopyCloud/pivotal-ui,pivotal-cf/pivotal-ui,pivotal-cf/pivotal-ui,npm/pivotal-ui,npm/pivotal-ui,npm/pivotal-ui