Cross repo Integration Tests

Our Continuous Integration framework runs some tests that verify that our dapps stay in sync with the main repo (agoric-sdk).

When incompatible changes are made, those cross-repo tests will fail until the tests are run against compatible releases. GitHub doesn't allow atomic merges across the separate repos, but we can require that there be a compatible release available before the merge.

To test a particular PR in agoric-sdk against a particular branch in dapp-encouragement, add a line like this to the top comment of the PR:

#dapp-encouragement-branch: zoe-release-0.7.0

similarly, in dap-encouragement, add this line to the check-in comment for the PR that is compatible with a branch in agoric-sdk:

#agoric-sdk-branch: zoe-release-0.7.0