extra/runner/run: Fix `killtree` on Linux

Commit 2e0e1097 changed `killtree` so that it worked on BSD/OSX, but
broke it on Linux. With Linux's GNU `ps`, the output of
`ps -o ppid=,pid=` is:

    ,pid=
     1921
     1922

instead of the desired:

     1921  1922
     1922  2282

`ps -o ppid= -o pid=` works with both GNU and BSD `ps`.

(`-o` selects which columns to display, and setting the column header to
blank (`<column>=<nothing>`) stops the header line being printed.)
extra/vm: Install the dependencies for extra/runner

So that I can use & test the runner in the VM.

I'm keeping the packages in the same order here as in `.travis.yml` in
the root of the git repository. The order I'm listing these packages is:

* Packages needed by core `stbt`
* Packages needed by `extra/runner`
* Packages needed to build the documentation and run the self-tests
* Miscellaneous packages