book: Update Clang references from 3.9 -> 4.0

Author: kulp

book/src/requirements.md

@@ -7,16 +7,13 @@ This page lists the requirements for running `bindgen` and how to get them.
 `bindgen` leverages `libclang` to preprocess, parse, and type check C and C++
 header files.
 
-It is recommended to use Clang 3.9 or greater, however `bindgen` can run with
+It is recommended to use Clang 4.0 or greater, however `bindgen` can run with
 older Clangs with some features disabled.
 
-* **If you are generating bindings to C,** 3.7 and 3.8 will probably work OK for
-you.
-
-* **If you are generating bindings to C++,** you almost definitely want 3.9 or
+* **If you are generating bindings to C++,** you almost definitely want 4.0 or
 greater.
 
-### Installing Clang 3.9
+### Installing Clang 4.0
 
 #### Windows
 
@@ -53,9 +50,9 @@ $ port install clang
 # apt install llvm-dev libclang-dev clang
 ```
 
-Ubuntu 16.10 provides the necessary packages directly. If you are using older
+Ubuntu 18.04 provides the necessary packages directly. If you are using older
 version of Ubuntu or other Debian-based distros, you may need to add the LLVM
-repos to get version 3.9. See http://apt.llvm.org/.
+repos to get version 4.0. See http://apt.llvm.org/.
 
 #### Arch
 
@@ -79,7 +76,7 @@ Add `export LIBCLANG_PATH=/usr/local/lib` to your profile.
 
 #### From source
 
-If your package manager doesn't yet offer Clang 3.9, you'll need to build from
+If your package manager doesn't yet offer Clang 4.0, you'll need to build from
 source. For that, follow the
 instructions [here](http://clang.llvm.org/get_started.html).