fix: toString PID number for writeFileSync (second arg requires s…

…tring) (#573)

Node 14 will no longer coerce incorrect types given to `writeFileSync`.

The error thrown:

```
TypeError: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received type number
```

src/services/process-service.ts

@@ -49,6 +49,6 @@ export default class ProcessService {
   }
 
   private writePidFile(pid: number) {
-    fs.writeFileSync(ProcessService.PID_PATH, pid)
+    fs.writeFileSync(ProcessService.PID_PATH, pid.toString())
   }
 }