Ignore .DS_Store when decrypting

[ocean90]

This would prevent an error about a not encrypted file, see the following example:

➜  appflow git:(master) make decrypt env=staging
[decrypt][example][staging]
ERROR! input is not encrypted for ~/.appflow/tenant/appflow-example/staging/.DS_Store
Decryption successful
Decryption successful
Decryption successful
Decryption successful
Decryption successful
Decryption successful
Decryption successful
Decryption successful

[ivomarino]

@89luca89 is on it.