Let travis check docs for stage1
auto merge of #13029 : FlaPer87/rust/master, r=alexcrichton

This should prevent lot of doc errors in Rust's buildbot and it shouldn't take long to run on travis. We could probably limit it to `std` but I preferred to just check all docs in this phase too.

@alexcrichton r?
Rewrite rc::Rc using cell::Cell

Since `Arc` has been using `Atomic`, this closes 12625.

Closes #12625.