ping command fails: TypeError: float() argument must be a string or a number
### Observed behavior

I think this fails silently in its own thread. Happened on Python 2.7, not sure why, but traceback is easy to fix.

### Expected behavior

n/a

### User-facing consequences

None, but ping doesn't happen

### Errors and logs

```
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/media/archive/code/kolibri/kolibri/utils/server.py", line 98, in run
    call_command("ping")
  File ".virtualenvs/kolibri/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 131, in call_command
    return command.execute(*args, **defaults)
  File ".virtualenvs/kolibri/local/lib/python2.7/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/media/archive/code/kolibri/kolibri/core/analytics/management/commands/ping.py", line 34, in handle
    interval = float(options.get("interval", DEFAULT_PING_INTERVAL))
TypeError: float() argument must be a string or a number

```


### Steps to reproduce

Sorry can't say for sure, was just reading a log.

### Context

Tell us about your environment, including:
 * Kolibri version: Either 0.7 or `develop` :)
 * Operating system: The best linux distro version 16.04
 * Browser: n/a

diff --git a/kolibri/core/analytics/management/commands/ping.py b/kolibri/core/analytics/management/commands/ping.py
index 3fc5451523..7feaa19a9b 100644
--- a/kolibri/core/analytics/management/commands/ping.py
+++ b/kolibri/core/analytics/management/commands/ping.py
@@ -31,9 +31,9 @@ class Command(BaseCommand):
 
     def handle(self, *args, **options):
 
-        interval = float(options.get("interval", DEFAULT_PING_INTERVAL))
-        checkrate = float(options.get("checkrate", DEFAULT_PING_CHECKRATE))
-        server = options.get("server", DEFAULT_PING_SERVER_URL)
+        interval = float(options.get("interval") or DEFAULT_PING_INTERVAL)
+        checkrate = float(options.get("checkrate") or DEFAULT_PING_CHECKRATE)
+        server = options.get("server") or DEFAULT_PING_SERVER_URL
 
         self.started = datetime.now()
 
['kolibri/core/analytics/management/commands/ping.py']