Error with logout function

## Description of the problem
When trying to logout an active session created from session cookies, an error occurs.

## Code to reproduce
```
session = fbchat.Session.from_cookies(cookie)
client = fbchat.Client(session=session)
session.logout()
```

## Traceback
```
Traceback (most recent call last):
File "", line 1, in
File "/.../fbchat-master/fbchat/_session.py", line 289, in logout
logout_h = re.search(r'name=\\"h\\" value=\\"(.*?)\\"', h_r.text).group(1)
AttributeError: 'NoneType' object has no attribute 'group'
```

## Environment information
- Python3.5
- `fbchat` 2.0.0a2

This error is not exclusive to v2. I'm getting same error using version 1.8.1 of fbchat. However I don't know if this error already existed or if a modification of fb handling of logout broke something (I've never used this functionality).