Use bash instead of zsh when no command specified

zsh isn't installed in the UML image so use bash instead.
Add support for redirecting ports with slirp

We can now redirect ports on the host to the guest VM using slirp.

To enable, specify HOST_IP and SLIRP_PORTS. For example, to enable access
to a web service running in the guest VM:

    HOST_IP=<eth0 IP> SLIRP_PORTS="80 443" ./run <some web service>

Specifying the host IP is required because slirp is bad at detecting it and
usually tries to bind to 127.0.0.1.
Add Travis CI build configuration