Change to gender-neutral language in help descriptions

The language for `keybase --help` refers to a user as “him” for the `track` and `encrypt` commands.
````
encrypt             verify a user's authenticity and optionally track him
track               verify a user's authenticity and optionally track him
````

Why assume users are men? This tells women that Keybase isn't for them. How about using the gender-neutral but [gramatically-correct](https://en.wikipedia.org/wiki/Singular_they) “them” instead.