Add details of which OS releases are tested in CI

We still find people trying to run test environments on older versions,
particularly Ubuntu Precise.

This release specifically mentions that we test on Ubuntu/Fedora 20
hosts with Trusty or Fedora20 guests.

Change-Id: Id1cd6c8771e67ce849f671b1148631ea7c3abcef
Merge "Add details of which OS releases are tested in CI"