Use Azure Pipelines for CI

[Brian Hou]

Pros:

• Faster than Travis (10 parallel builds, rather than 5; VMs may also just be better)
• Longer build time limit (seems to be 6 hours, rather than 50 minutes)
• Default Ubuntu image is Xenial

Cons:

• We're currently using Travis

Before transitioning over, we should try setting up Azure Pipelines alongside Travis by making our scripts CI agnostic.

[Ethan K. Gordon]

Addressed by #598