Auto Discovery for HS100 plugs not working
<!-- Provide a general summary in the Title above -->

### Expected Behavior
<!-- If you're describing a bug, tell us what should happen -->
<!-- If you're suggesting a change/improvement, tell us how it should work -->
When homebridge starts it should discover the existing plugs on the network.


### Current Behavior
<!-- If describing a bug, tell us what happens instead of the expected behavior -->
<!-- If suggesting a change/improvement, explain the difference from current behavior -->
Two plugs configured and working in the Kasa mobile application are not discovered. I have already tried deleting the cached and persisted accessories. Everything was working fine till a couple of days back when the plugs stopped responding from the 'Home' app on the phone.

### Things I have tried
As in a similar issue (#47) , I have tried the following with no joy:

- [x] Clear  ```.homebridge/accessories``` and ```.homebridge/persist```  folders (as per your [suggestion](https://github.com/plasticrake/homebridge-tplink-smarthome/issues/7#issuecomment-267184245) on issue #7 )
- [x] Reset both TPLink plugs and set them up again via the Kasa app
- [x] Uninstall homebridge and all related files and reinstall
- [x]  Update TP-Link devices IP from auto to manual IP's & update config file to match (as seen in the second version of the config.json below)
- [x] Downgrade node version (this worked for the creator of issue #7 as seen in [this](https://github.com/plasticrake/homebridge-tplink-smarthome/issues/7#issuecomment-269109577) comment)
- [x] **Try a router restart!**
 
### Steps to Reproduce (for bugs)
<!-- Please add a series of steps to reproduce the problem. -->
- [x] Kill all instances of ```homebridge```
- [x] Delete any instances of homebridge already associated in the iOS Home app. 
- [x] Start homebridge
- [x] Wait for 'em HS100 plugs to be discovered
- [x] Rejoice!


### Versions
<!-- Please run commands and include versions below. -->
- **Node**:       `v9.4.0`
- **Homebridge**: `0.4.38`
- **OS**:         `macOS 10.12.6`

### Configuration
<!-- Please include the TplinkSmarthome platform section of your config.json file.
Paste between the the two ``` lines below.
-->
```json
    "platforms": [{
        "platform": "TplinkSmarthome",
        "name": "TplinkSmarthome",
        "devices": [
            { "host": "192.168.1.96" },
            { "host": "192.168.1.98" }
        ]
    }]
```

OR,

```
    "platforms": [{
        "platform": "TplinkSmarthome",
        "name": "TplinkSmarthome"
    }]
```

### Homebridge Log / Command Output

Besides the output of ```homebridge``` below, this might be useful as well (as I recall you asking for this in another issue):

```bash
$ tplink-smarthome-api search
Searching...
startDiscovery({ discoveryInterval: 2000, discoveryTimeout: 5000 })

```


<!-- Paste relevant output between the two ``` lines below
  - Remove any sensitive information, passwords, etc.
  - If output is long:
    - Please extract only the relevant portions if possible (remove other plugin's output, etc)
    - Consider creating a gist, etc
  - However, please include the beginning of the log where the homebridge initialization happens if possible
-->

With either version of the config.json above:
```
$ homebridge
[27/02/2018, 10:46:22] Loaded plugin: homebridge-tplink-smarthome
[27/02/2018, 10:46:22] Registering platform 'homebridge-tplink-smarthome.TplinkSmarthome'
[27/02/2018, 10:46:22] ---
[27/02/2018, 10:46:22] Loaded config.json with 0 accessories and 1 platforms.
[27/02/2018, 10:46:22] ---
[27/02/2018, 10:46:22] Loading 1 platforms...
[27/02/2018, 10:46:22] [TplinkSmarthome] Initializing TplinkSmarthome platform...
[27/02/2018, 10:46:22] [TplinkSmarthome] homebridge-tplink-smarthome v3.11.0, node v9.4.0, homebridge v0.4.38
Setup Payload:
X-HM://0023HO8UXD31P
Scan this code with your HomeKit app on your iOS device to pair with Homebridge:
<redacted>
Or enter this code with your HomeKit app on your iOS device to pair with Homebridge:
<redacted>
[27/02/2018, 10:46:22] Homebridge is running on port 51826.
```

(Off-topic : The issue template is quite nice 👍 )