Prolific Queue v20.0.0

• Update LICENSE for 2020. Closes #1308.
• Upgrade prospective to ^0.2.0. Closes #1307.
• Upgrade destructible to ^6.0.0. Closes #1306.
• Upgrade staccato to ^12.0.0. Closes #1305.
• Upgrade prolific.collector to ^12.0.0. Closes #1304.
• Upgrade prolific.watcher to ^0.2.0. Closes #1303.
• Upgrade rimraf to 3.0.2. Closes #1302.
• Upgrade duplicitous to ^0.2.0. Closes #1301.
• Upgrade proof to ^9.0.1. Closes #1300.

Closes #1309.