Temporary(?) `_verman` => `_version` hack

Blech.  Need to figure out how versions should be inherited in child
processes.  Motivating case:

```sh
 # in .zshenv, set a default Ruby
_verman ruby use 2.1.1
 # then:
cd {some project using ruby 2.1.2}
command ruby --version
 # => ruby 2.1.2p95 ...
 # but:
bundle exec ruby --version
 # => error: Your Ruby version is 2.1.1, but ...
```

For now, always export the version, and don't set up an overriding
version in the child process.  Keep `_verman` as the main wrapper for
`verman`+eval.  Use `_version` as this new "set up a version, but don't
override unless forced" function.
Add auto-cd function to set Ruby version

- TODO: handle other langs and/or move this into Verman