Fixed cache invalidation issue

Steps to reproduce: 
1. create a smart snippet 
2. add that smart snippet to a page 
3. preview the page 
4. wait for a couple of minutes 
      more than settings.CACHES['default']['TIMEOUT'] (currently 1m) and less than SMARTSNIPPETS_CACHING_TIME (1h by default) 
5. change the smart snippet 
6. preview the page again 

Expected: 
change made at step 5 is visible 

Actual: 
the change is not there