Integrate a tool to block PRs until downstream packages are pushed.

[adivekar-utexas]

fmcore has a hard dependency on bears, and this causes problem when raising PRs that require changes in both packages.
An example is here: #19, where to fix this bug, both bears and fmcore needed to be updated.

Some potential options for better merging experience:

• Multi-PR merge tools:
  1. Mergify: https://github.com/mergifyio
  2. Multi-gitter: https://github.com/lindell/multi-gitter
• Failing tests:
  1. Make fmcore test cases install from latest commit on bears package, rather than latest release.