added .json to end of cache file name

[0xStabby]

Fix to this issue: #11

[0xEnrico]

This pull request seems to me to get .json.cache and not .cache.json as it's needed by the CM.
Should be something like:

cache_filename = os.path.join('.json', os.path.join('.cache', args.env + "-" + args.cache_name))

to get the correct behaviour.
But let me have a quick look to support both new and old behaviour.

[0xEnrico]

Sorry, you are right, it's correct, but still, it would be nice to support both old and new filename. I'll have a look, should not be too difficult