Complex CSS selectors fail

Complex CSS selectors (as identified by [SelectorGadget](http://selectorgadget.com/)) often fail:

```
test_that("complex CSS", {
  expect_is(s <- html_session("http://testing-ground.scraping.pro/"), "session")
  expect_is(s %>% follow_link(".caseblock:nth-child(1) a", "css"), "session")
})
```
