Pokemon Go: How to evolve Eevee into a Vaporeon, Jolteon or Flareon

As discovered by people on Reddit, and now confirmed several times through different users, renaming your Eevee to Rainer, Sparky or Pyro before it's evolved will give you the corresponding evolution. For example, if you're after a Flareon, you'd rename your Eevee to Pyro, save and restart your game, and then evolve it.