Increase wait times for window_opened_by spec due to slow Travis CI

To debug Travis failures I added once the following to `window_opened_by` method when running tests on Travis:
```ruby
require 'benchmark'
time = Benchmark.realtime do
  driver.window_handles
end
puts time * 1000
```

On my machine it executes from 3 to 140 ms. On Travis it took ~760 ms once - https://travis-ci.org/jnicklas/capybara/jobs/33410167#L492 so spec failed.

So I changed that failing spec so it won't fail if takes so long. Probably it would be needed to increase it even more.