Afterburn v4.2.0

Changes:

• providers: fetch instance types for all providers
• providers: add Alibaba Cloud (aliyun)
• systemd: allow sshkeys service on aliyun
• tests/gcp: avoid flake
• travis: bump minimum toolchain
• metadata: add dyn to remove warning

SHA-256 digests:

• crate: 41c24eccc74f1b21e26cf564cc223c0dfc16e47315244439be732c2984224a73
• vendor: ccfa6332a25961fc4a436ceb48212ebdafeb9eedf316d8410788bf5cd5d69881