Fix for trailing tasks with userinput

I had some issues with this plugin and grunt-prompt.

All characters were displayed twice after keystroke.

Example:
```javascript
…
prompt: {
	input: {
		options: {
			questions: [{
				config: 'echo.field',
				type: 'input',
				message: 'input: ',
				default: 'default'
			}]
		}
	}
}
…
```

```bash
# with fix
 $ grunt prompt:input

Running "lockfile:buildprocess" (lockfile) task
>> Lockfile established

Running "prompt:input" (prompt) task
[?] input: abc

Done, without errors.




# without fix
 $ grunt prompt:input
11260: "started process with id 11260"

Running "lockfile:buildprocess" (lockfile) task
>> Lockfile established

Running "prompt:input" (prompt) task
[?] input: (default) aabbcc
[?] input: abc

```