Fix gemspec files attribute on Ubuntu
Fix gemspec files attribute on Ubuntu

579a0e5 refactor changed Dir.glob to `git ls-files -- {bin,lib}/*...`
Ubuntu's /bin/sh is /bin/dash which does not support brace expansions over strings.

Change-Id: I862e4704c5d2248482e284d3e04e6c5f4d6db7b2

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh