Added libbz2-dev to Debian based prerequisites

Encountered this when building for the first time today:

```
note: /usr/bin/ld: cannot find -lbz2
collect2: error: ld returned 1 exit status

error: aborting due to previous error
Could not compile `servo`.
```

Installing the libbz2-dev packaged fixed it. Added to the list of packages to install on Debian based linuxes in README.md