Invoke IO.popen correctly for Ruby 1.9.3
Invoke IO.popen correctly for Ruby 1.9.3

Before this change, tests in gitlab_projects_spec were passing on Ruby
2.0.0 but not on 1.9.3.

bozaro/gitlab-shell,kotaro-dev/gitlab-shell-1-9-4,dpursehouse/gitlab-shell,cui-liqiang/gitlab-shell,dwrensha/gitlab-shell,dwrensha/gitlab-shell,gitlabhq/gitlab-shell,achako/gitlab-shell,Jadaw1n/gitlab-shell,yonh/gitlab-shell,fscherwi/gitlab-shell,sonalkr132/gitlab-shell,kotaro-dev/gitlab-shell-1-9-4,fscherwi/gitlab-shell,dpursehouse/gitlab-shell,feedhenry/gitlab-shell,gitlabhq/gitlab-shell,cui-liqiang/gitlab-shell,yonh/gitlab-shell,gitlabhq/gitlab-shell,copystudy/gitlab-shell,feedhenry/gitlab-shell,sonalkr132/gitlab-shell,copystudy/gitlab-shell,Jadaw1n/gitlab-shell,achako/gitlab-shell,bozaro/gitlab-shell