Go to the Eevee you want to evolve in your Pokemon list, hit the pen icon next to its name and then rename it according to the Eeveelution you want - see below.

• Sparky = Jolteon
• Rainer = Vaporeon
• Pyro = Flareon

Quit out of the app and restart it, just to make sure the name's registered with the Pokemon Go servers and then evolve your Eevee. If you did it right you will get the desired evolution!