Nit: LLVM & Clang latest version is 4.0

[kirillbobyrev]

Small nit: since latest Clang version is 4.0 it's nice to reflect this in the documentation.

Also, I couldn't find anything, but there might be any hard-coded check that Clang version matches "3.X" anywhere in the build system; if there is one, it'd be great to bump that one too.

[rust-highfive]

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[alexcrichton]

@bors: r+

[bors]

📌 Commit fac91c5 has been approved by alexcrichton

[bors]

🔒 Merge conflict

[bstrie]

I've rebased this branch due to a submodule conflict that invalidated many PRs.

@bors r=alexcrichton

[bors]

📌 Commit 71020e3 has been approved by alexcrichton