Simplified expiry date form code.

No need for Rails's weird `select_date` helper when we just need a
year.
Upgraded to Poltergeist 1.5.0.

Poltergeist is much faster, but we ran into some intermittent test
failures. The problem seems to be a race condition where we look
for the "Logout" link before the page has refreshed. Tried a few
things to make it work better, but it still fails sometimes. We're
going to live with it for now, and maybe try Capybara 2.2.
Refactor: set expiry fields only if paying by card.

We were setting those fields even if you paid with a bank account.
Refactor: unduplicated code in billings#create/update.

Most of this code should be implemented by the model, and/or just by
setting @payment.attributes = params[:payment]. But this is a safe
improvement for now.