Update to java8 and install graphviz on dev VM
Update to java8 and install graphviz on dev VM

We already use graphviz to generate some diagrams, and I want
to generate some more.

We've already updated Java on CI:
https://github.com/alphagov/govuk-puppet/pull/7449

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet