Add Vagrant configuration.

capybara-webkit has various development dependencies which can be a little
cumbersome to manage. With vagrant they can all be nicely bundled up in a VM and
installed automatically.
Fix typo: change Postion to Position
Forward stderr from server to stderr, drop stdout

We don't actually write anything to stdout. Also rename the stdout
option to stderr, since that's where we forward the output.
Don't forward stderr to nil

Fixes #455.