Refactor: remove ContinueWithError (#630)

* Update util.go

Remove Continue with error function

* Update cmd/util.go

Co-authored-by: Liz Rice <liz@lizrice.com>

* Update util.go

* Update util.go

Remove unnecessary ')'

* Update util.go

removed fmt.Fprintf(os.Stderr, "%s: %s", cmd.Args, err) since it wasn't suppose to print.

* Update util.go

* Update .travis.yml

option --no-ri and --no-doc are deprecated we have to use --no-document instead.
https://github.com/rubygems/heroku-buildpack-bundler2/pull/1#issuecomment-451654992



Co-authored-by: Liz Rice <liz@lizrice.com>