[Libraries] Limit built branches to master and PRs
[Libraries] Limit built branches to master and PRs

For other branches we don't have a spec in GitVersion.yml, so a
build will likely fail because gitversion can't detect the version
number.

Change-Id: Id406298db99a759503b0b04edf7dca967ec9a6c0

sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts,sillsdev/ci-builder-scripts