Update README.md

[whikloj]

Build badge was still pointing at the 1.x branch

GitHub Issue: n/a

What does this Pull Request do?

Updates the CI build badge to show the correct state on the README

What's new?

• Does this change require documentation to be updated? No
• Does this change add any new dependencies? No
• Does this change require any other modifications to be made to the repository (i.e. Regeneration activity, etc.)? No
• Could this change impact execution of existing code? No

How should this be tested?

Se the correct (read: passing) build badge on the Alpaca README

Interested parties

@Islandora/committers