encrypt only the value if key is an empty string

this pull request only modifies one line in your code to change the behavior if key is an empty string

```
$ ## example usage with empty key string
$ travis-encrypt -r kaizhu256/utility2 -k "" -v "heroku_api_key_secret"
# undefined
+UOJknUje0te1+54BFtDQi+1jFBTDfFI7qYSV+tcDMkCeQJ7K/lh4y45c8w+I/OE0BL7eyL1MowiMf5oQnSlDg==
```