Fix unstable spec on circle ci

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Fix #142 (maybe)

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output = `#{gem} list`
p $?.exitstatus #=> 1
p output #=> looks successfully output

system("which #{gem}")
output = `#{gem} list`
p $?.exitstatus #=> 0
p output #=> looks successfully output

I'm confusing this behavior...
Maybe interpret with --seed 34923

https://circleci.com/gh/fluent/fluentd-ui/624

[uu59]

Pressed CircleCI rebuild button with 5 times, then all builds are green.
Looks test is stable