* fix: use Bash for script
Using `/bin/sh` lead to multiple warnings that could result in errors in some POSIX environments. See [SC2039](https://github.com/koalaman/shellcheck/wiki/SC2039).
* In POSIX sh, [[ ]] is undefined.shellcheck(SC2039)
* In POSIX sh, string replacement is undefined.shellcheck(SC2039)
* Using /bin/sh leads to authentication errors for my CI | https://travis-ci.org/MaximDevoir/probot/builds/631990067
* fix: get latest tag for `VERSION`
Previous commit would retrieve an older tag