Do not propose unstable features on autocomplete for stable toolchain

In the project with stable toolchain used, `Some(22).ex<|>` proposes `expect_none` [method](https://doc.rust-lang.org/std/option/enum.Option.html#method.expect_none) which is unstable currently.

![image](https://user-images.githubusercontent.com/2690773/73848680-3ea7c080-4831-11ea-9e80-7c5ac3a02a56.png)

I would expect RA not to propose unstable APIs when a stable toolchain is used.