Remove pre-Go1.9 fallback code

Go 1.12beta1 is out and even Appengine's oldest runtime is Go 1.9,
so get rid of pre-1.9 fallback code and always use math/bits.

rotate.go

diff --git a/rotate.go b/rotate.go
deleted file mode 100644
index f3eac5e..0000000
--- a/rotate.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// +build !go1.9
-
-package xxhash
-
-// TODO(caleb): After Go 1.10 comes out, remove this fallback code.
-
-func rol1(x uint64) uint64  { return (x << 1) | (x >> (64 - 1)) }
-func rol7(x uint64) uint64  { return (x << 7) | (x >> (64 - 7)) }
-func rol11(x uint64) uint64 { return (x << 11) | (x >> (64 - 11)) }
-func rol12(x uint64) uint64 { return (x << 12) | (x >> (64 - 12)) }
-func rol18(x uint64) uint64 { return (x << 18) | (x >> (64 - 18)) }
-func rol23(x uint64) uint64 { return (x << 23) | (x >> (64 - 23)) }
-func rol27(x uint64) uint64 { return (x << 27) | (x >> (64 - 27)) }
-func rol31(x uint64) uint64 { return (x << 31) | (x >> (64 - 31)) }

rotate19.go

diff --git a/rotate19.go b/rotate19.go
deleted file mode 100644
index b99612b..0000000
--- a/rotate19.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// +build go1.9
-
-package xxhash
-
-import "math/bits"
-
-func rol1(x uint64) uint64  { return bits.RotateLeft64(x, 1) }
-func rol7(x uint64) uint64  { return bits.RotateLeft64(x, 7) }
-func rol11(x uint64) uint64 { return bits.RotateLeft64(x, 11) }
-func rol12(x uint64) uint64 { return bits.RotateLeft64(x, 12) }
-func rol18(x uint64) uint64 { return bits.RotateLeft64(x, 18) }
-func rol23(x uint64) uint64 { return bits.RotateLeft64(x, 23) }
-func rol27(x uint64) uint64 { return bits.RotateLeft64(x, 27) }
-func rol31(x uint64) uint64 { return bits.RotateLeft64(x, 31) }

xxhash.go

diff --git a/xxhash.go b/xxhash.go
index 9a0e59c..b0e5440 100644
--- a/xxhash.go
+++ b/xxhash.go
@@ -4,6 +4,7 @@
 
 import (
 	"encoding/binary"
+	"math/bits"
 )
 
 const (
@@ -173,3 +174,12 @@
 	acc = acc*prime1 + prime4
 	return acc
 }
+
+func rol1(x uint64) uint64  { return bits.RotateLeft64(x, 1) }
+func rol7(x uint64) uint64  { return bits.RotateLeft64(x, 7) }
+func rol11(x uint64) uint64 { return bits.RotateLeft64(x, 11) }
+func rol12(x uint64) uint64 { return bits.RotateLeft64(x, 12) }
+func rol18(x uint64) uint64 { return bits.RotateLeft64(x, 18) }
+func rol23(x uint64) uint64 { return bits.RotateLeft64(x, 23) }
+func rol27(x uint64) uint64 { return bits.RotateLeft64(x, 27) }
+func rol31(x uint64) uint64 { return bits.RotateLeft64(x, 31) }