age: unstable-2020-03-25 -> v1.0.0-beta4

Bumping to upstream release from 2020-06-28. This skips 1.0.0-beta3,
which was released on the same day and contained a bug.

Upstream changelog:

189041b6 (tag: v1.0.0-beta4) age: move package from filippo.io/age/age to filippo.io/age 🤦♂
e6093596 (tag: v1.0.0-beta3) age,agessh,armor: unleash public API 💥🦑
33355dcc internal/age: unexport NewX25519Recipient and NewX25519Identity
9a08b7e6 internal/age,internal/armor: add examples
9b83d948 internal/age: surface format.Recipient as type Stanza
c9a35c07 internal/agessh: move EncryptedSSHIdentity out of cmd/age
7d608d12 .github/workflows: add rage interop tests trigger (#125)
6782356e internal/age: add some docs and polish API
08546656 internal/format: fix a nasty bufio.Reader nesting bug
292c3aae internal/agessh: new package
b32ea4c1 cmd/age: add a TODO about not dumping decrypted binary to the terminal
c7c7f187 internal/armor: new package
a7c4274d internal/age: remove EncryptWithArmor and armor support in Decrypt
7088a732 internal/age: unexport SSHFingerprint