Use goreleaser to build releases and also publish Homebrew formula

Created by: mrnugget

I discovered GoReleaser a few days ago when I found out that Github's new gh CLI is automatically released as a Homebrew formula: https://github.com/github/homebrew-gh

That would be really nice to have. Now, I'm not sure whether the config here works as intended.

Doing a dry-run release locally seems to work:

 goreleaser --snapshot --skip-publish --rm-dist

   • releasing using goreleaser 0.126.0...
   • loading config file       file=.goreleaser.yml
   • RUNNING BEFORE HOOKS
      • running go mod download
      • running go mod tidy
   • LOADING ENVIRONMENT VARIABLES
      • pipe skipped              error=publishing is disabled
   • GETTING AND VALIDATING GIT STATE
      • releasing 3.10.8, commit f907940b61d30be8c115c8f0940e48c9dda556e8
      • pipe skipped              error=disabled during snapshot mode
   • PARSING TAG
   • SETTING DEFAULTS
      • LOADING ENVIRONMENT VARIABLES
      • SNAPSHOTING
      • GITHUB/GITLAB/GITEA RELEASES
      • PROJECT NAME
      • BUILDING BINARIES
      • ARCHIVES
      • LINUX PACKAGES WITH NFPM
      • SNAPCRAFT PACKAGES
      • CALCULATING CHECKSUMS
      • SIGNING ARTIFACTS
      • DOCKER IMAGES
      • ARTIFACTORY
      • BLOB
      • HOMEBREW TAP FORMULA
         • optimistically guessing `brew[0].installs`, double check
      • SCOOP MANIFEST
   • SNAPSHOTING
   • CHECKING ./DIST
      • --rm-dist is set, cleaning it up
   • WRITING EFFECTIVE CONFIG FILE
      • writing                   config=release/config.yaml
   • GENERATING CHANGELOG
      • pipe skipped              error=not available for snapshots
   • BUILDING BINARIES
      • building                  binary=/Users/thorstenball/work/src-cli/release/src-cli_windows_amd64/src.exe
      • building                  binary=/Users/thorstenball/work/src-cli/release/src-cli_darwin_amd64/src
      • building                  binary=/Users/thorstenball/work/src-cli/release/src-cli_linux_amd64/src
   • ARCHIVES
      • creating                  archive=release/src-cli_3.10.8-SNAPSHOT-f907940_windows_amd64.tar.gz
      • creating                  archive=release/src-cli_3.10.8-SNAPSHOT-f907940_darwin_amd64.tar.gz
      • creating                  archive=release/src-cli_3.10.8-SNAPSHOT-f907940_linux_amd64.tar.gz
   • LINUX PACKAGES WITH NFPM
   • SNAPCRAFT PACKAGES
   • CALCULATING CHECKSUMS
      • checksumming              file=src-cli_3.10.8-SNAPSHOT-f907940_darwin_amd64.tar.gz
      • checksumming              file=src-cli_3.10.8-SNAPSHOT-f907940_linux_amd64.tar.gz
      • checksumming              file=src-cli_3.10.8-SNAPSHOT-f907940_windows_amd64.tar.gz
   • SIGNING ARTIFACTS
      • pipe skipped              error=artifact signing is disabled
   • DOCKER IMAGES
      • pipe skipped              error=docker section is not configured
   • PUBLISHING
      • pipe skipped              error=publishing is disabled
   • release succeeded after 2.71s

But that also includes the snapshots and I'm mostly concerned about the Travis CI configuration.

I moved the secret token from the deply configuration to the global env vars, since goreleaser expects a GITHUB_TOKEN env var, but since I don't have an encryption key, I'm not sure what's in the secret (cc @slimsag)