[bug 1032967] Handle unsupported language pairs

Previously we looked at the Gengo service languages list to see if the
src language was supported and went with that. However, that wasn't
correct and created problems. We really need to see if the (src, dst)
pair is supported.

This fixes that.
Merge pull request #318 from willkg/1032967-language-service-not-supported

[bug 1032967] Handle unsupported language pairs
[bug 1026503] First pass at vagrantification

* adds vagrant stuff and provision scripts
* first pass at overhauling docs to use Vagrant rather than explicitly
  walk through how to set up a Fjord development environment
Continued working on docs; ditched hacking howto
Assume VB mismatch and codify the fix in the docs

Ian pointed out that most people will have a mismatch, so we should be
pro-active about that and address it in the docs before provisioning.
This adds the necessary bits to the docs plus it removes the note from
the provisioning script.
Merge pull request #314 from willkg/1026503-vagrantify

[bug 1026503] First pass at vagrantification