update and fix specs

so, this was hard but i'm getting all green now.

different kind of fixes was needed, some sanity checking is very welcome, it's the first time i look into this extension code.

i switched js driver to selenium as that's the only way to make tests green.
there's a chance it's my environment fault, but i also tried different poltergeist and capybara versions.
looks like the redirect from spree to paypal doesn't work at all.

i started with `2-3-stable` branch as that's what i'm using now.