Small change to the base_url: Using https instead of http

Hi Ryan, 

I am using motionless in one my projects. Unfortunately, the created static images are marked as unsafe since the base_url was set up to http://...
In this pull request, I changed the base_url to 'https://maps.google.com/maps/api/staticmap?'.

Could you please merge and push the new master to into the pip package? Thank you.