migrate: detect version 2.0.1

Without this code a second start will crash:

```
$ ./bin/etcd -name foobar --data-dir=foobar
2015/03/18 18:06:28 starter: detect etcd version 2.0.1 in foobar
2015/03/18 18:06:28 starter: unhandled etcd version in foobar
panic: starter: unhandled etcd version in foobar

goroutine 1 [running]:
log.Panicf(0x594770, 0x25, 0x208927c70, 0x1, 0x1)
	/usr/local/go/src/log/log.go:314 +0xd0
github.com/coreos/etcd/migrate/starter.checkInternalVersion(0x20889a480, 0x0, 0x0)
	/Users/philips/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/migrate/starter/starter.go:160 +0xf2f
github.com/coreos/etcd/migrate/starter.StartDesiredVersion(0x20884a010, 0x3, 0x3)
	/Users/philips/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/migrate/starter/starter.go:77 +0x2a9
main.main()
	/Users/philips/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/main.go:46 +0x25e

goroutine 9 [syscall]:
os/signal.loop()
	/usr/local/go/src/os/signal/signal_unix.go:21 +0x1f
created by os/signal.init·1
	/usr/local/go/src/os/signal/signal_unix.go:27 +0x35
```

migrate/starter/starter.go

@@ -113,6 +113,8 @@ func checkInternalVersion(fs *flag.FlagSet) version {
 	switch dataver {
 	case wal.WALv2_0:
 		return internalV2
+	case wal.WALv2_0_1:
+		return internalV2
 	case wal.WALv2_0Proxy:
 		return internalV2Proxy
 	case wal.WALv0_4: