Don't require rspec/autorun

Not necessary and causes issues with some test runners, e.g. zeus
Don't assume HMRC is the only organisation
Simplify contacts factory

Although it's not a validation, we would always expect a contact to have
an organisation. The :organisation trait was never used and also broken
anyway (I'm assuming because it had the same name as an association on
the model), and the little dance to set all the contact's groups to be
owned by the same org is unnecessary and likely to cause confusion if
someone explicitly sets a gorup that isn't owned by the same org.
Correctly scope contacts to organisations

All contacts were being listed for all organisations, regardless of the
organisation they actually belonged to.
Merge pull request #130 from alphagov/fix-contacts-scoping-bug

Fix contacts scoping bug