Update dependencies

vendor/github.com/cespare/xxhash/v2/README.md

@@ -28,6 +28,18 @@ func (*Digest) Sum64() uint64
 This implementation provides a fast pure-Go implementation and an even faster
 assembly implementation for amd64.
 
+## Compatibility
+
+This package is in a module and the latest code is in version 2 of the module.
+You need a version of Go with at least "minimal module compatibility" to use
+github.com/cespare/xxhash/v2:
+
+* 1.9.7+ for Go 1.9
+* 1.10.3+ for Go 1.10
+* Go 1.11 or later
+
+I recommend using the latest release of Go.
+
 ## Benchmarks
 
 Here are some quick benchmarks comparing the pure-Go and assembly