From a reddit post:
"Hey All,
It would seem that there is in fact a way to force an evolution of your Eevee. The Eevee Brothers from the original anime were named Rainer, Pyro, Sparky and they owned a Vaporeon, Flareon and Jolteon respectively. If you nickname your Eevee either Rainer, Pyro or Sparky, your Eevee will evolve into that desired Eeveelution!
Sparky = Jolteon
Rainer = Vaporeon
Pyro = Flareon
I have tried it on a few Eevees and so have friends, and we've seen a 100% success rate. Feel free to give it a try and post back here with results!"