Unison 2.51.3 and arm64 support

Unison upgraded to 2.51.3

• builds/tags for 2.51.4_rc2 exist too

Add unison native support for Apple M1 (experimental)

To use the ARM64 build https://docker-sync.readthedocs.io/en/latest/getting-started/configuration.html#sync-strategy-image to eugenmayer/unison:2.51.3-4.12.0-ARM64

Misc

• several docs, typos and glitches fixes by contributors. Please cheer them!
• Minumum ruby version is 2.4
• Bundler upgraded to 2.x
• Thor upgraded to 1.1
• upgraded activesupport to 5.2