Add Apple M1 Build Arch (& other arm64)

[jblz]

Now that go 1.16 has dropped, we can add darwin/arm64 to our build targets.

This change:

• Bumps the go version to 1.16.x in the Travis CI config
• Splits out the darwin platform into its own gox call (darwin/386 is not a valid GOOS/GOARCH pair)
• Adds a barebones go.mod file (to get past cannot find main module error in go vet)
• Adds a comment to the Replacement struct (to satisfy the lint job)
• Runs the test file through gofmt -s (to satisfy the fmt job)

Discussion

https://a8c.slack.com/archives/CKUJZT08Y/p1614186371037300?thread_ts=1614128274.023800&cid=CKUJZT08Y

To Test

• Install go 1.16
• Clone this repo
• make
  • The build should complete without error
  • ls build should contain valid gzipped binaries for all supported platform / architecture pairs.

Test on an M1 Mac

• curl -L -o - https://github.com/Automattic/go-search-replace/releases/download/0.0.6-pre/go-search-replace_darwin_arm64.gz | gunzip > /tmp/go-search-replace
• chmod +x /tmp/go-search-replace
• echo "You can do anything at https://zombo.com" | /tmp/go-search-replace "zombo.com" "wpvip.com"
  • Should print:

    You can do anything at https://wpvip.com

Fixes #27

[jblz]

@joshbetz looks like go 1.16 shipped, but I couldn't get this working by just adding the arch. I tried splitting into a separate gox command, but no 🎲. Any ideas?