option 2 - make key param optional in travis-encrypt

second option is a more involved patch, which makes the key param entirely optional

```
$ ## example usage with no key param
$ travis-encrypt -r kaizhu256/utility2 -v "heroku_api_key_secret"
# undefined
+UOJknUje0te1+54BFtDQi+1jFBTDfFI7qYSV+tcDMkCeQJ7K/lh4y45c8w+I/OE0BL7eyL1MowiMf5oQnSlDg==
```