Fix "Running A Test Index" doc
Merge pull request #363 from pquentin/patch-1

Add LD_LIBRARY_PATH specification to docs.

The vagrant provisioning script tries to install the lib globally (https://github.com/mozilla/dxr/blob/master/vagrant_provision.sh#L71-L74), but it doesn't work unless the lib is already built, and it can't be built until the vagrant box is up.