Update the docs on min Swift version required.

Author: thomasvl

Documentation/CONFORMANCE_TESTS.md

@@ -16,7 +16,7 @@ these tests regularly.
 
 ## Preparation
 
-The conformance test suite requires Swift 3.2, standard command-line
+The conformance test suite requires Swift 4.0, standard command-line
 tools such as make and awk, and a full source checkout of
 [Google's protobuf project](https://github.com/protocolbuffers/protobuf).
 

Documentation/PLUGIN.md

@@ -19,7 +19,7 @@ Swift runtime library to your project.
 
 To use Swift with Protocol buffers, you'll need:
 
-* A recent Swift 3.2 compiler that includes the Swift Package Manager.
+* A recent Swift 4.0 compiler that includes the Swift Package Manager.
   We recommend using the latest release build from
   [Swift.org](https://swift.org) or the command-line tools included
   with the latest version of Xcode.

README.md

@@ -90,7 +90,7 @@ your project as explained below.
 
 To use Swift with Protocol buffers, you'll need:
 
-* A Swift 3.2 or later compiler (Xcode 9.0 or later).  Support is included
+* A Swift 4.0 or later compiler (Xcode 9.1 or later).  Support is included
 for the Swift Package Manager; or using the included Xcode project. The Swift
 protobuf project is being developed and tested against the latest release
 version of Swift available from [Swift.org](https://swift.org)