build: bump to Go 1.10

Bumps the build to use Go 1.10, which allows us to drop the forked tar
package.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

archive/tar_unix.go

@@ -19,13 +19,13 @@
 package archive
 
 import (
+	"archive/tar"
 	"context"
 	"os"
 	"sync"
 	"syscall"
 
 	"github.com/containerd/continuity/sysx"
-	"github.com/dmcgowan/go-tar"
 	"github.com/opencontainers/runc/libcontainer/system"
 	"github.com/pkg/errors"
 	"golang.org/x/sys/unix"