all: increase minimum supported version to go1.9 (#742)

Change-Id: I2a4b279bd969483f4136d02b1e224ee342726b35
Cherry-Pick: github.com/golang/protobuf@75dceb112b174be156fa9952e66d3e99945572b4
Original-Author: Joe Tsai <joetsai@digital-static.net>
Reviewed-on: https://go-review.googlesource.com/c/151434
Reviewed-by: Damien Neil <dneil@google.com>

README.md

@@ -7,7 +7,7 @@ Google's data interchange format.
 Copyright 2010 The Go Authors.
 https://github.com/golang/protobuf
 
-This package and the code it generates requires at least Go 1.6.
+This package and the code it generates requires at least Go 1.9.
 
 This software implements Go bindings for protocol buffers.  For
 information about protocol buffers themselves, see

protoc-gen-go/grpc/grpc.go

@@ -55,7 +55,7 @@ const generatedCodeVersion = 4
 // Paths for packages used by code generated in this file,
 // relative to the import_prefix of the generator.Generator.
 const (
-	contextPkgPath = "golang.org/x/net/context"
+	contextPkgPath = "context"
 	grpcPkgPath    = "google.golang.org/grpc"
 )