Password on command line insecure warning

Started seeing this warning recently after upgrading to a newer version of MySQL:
```
mysqldump: [Warning] Using a password on the command line interface can be insecure.
```

Here's the version of mysqldump I'm using on Ubuntu 16.04:
```
$ mysqldump --version
mysqldump  Ver 10.13 Distrib 5.7.20, for Linux (x86_64)
```