Pin SleekXMPP to 1.3.2

1.3.3 is broken as of fritzy/SleekXMPP#461. Someone seems to have forget to adapt the requirements after the first deployment in 2017 of 7e9e016.

Fore reference the exception is

```
ValueError

time data '190530023040Z' does not match format '%Y%m%d%H%M%SZ'
```

extra/requirements/production.txt

@@ -31,6 +31,6 @@ pyzmq==16.0.3
 raven==6.3.0
 redis==2.10.6 # django-redis requires redis>=2.10, but redis 3.x is incompatible with django-redis==4.4.4
 requests==2.20.0
-SleekXMPP==1.3.3
+SleekXMPP==1.3.2 # 1.3.3 breaks, see https://github.com/fritzy/SleekXMPP/issues/461
 transifex-client==0.12.5
 Werkzeug==0.12.2