Fix Bundler related error on Travis CI#57

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kou wants to merge 2 commits intomethodmissing:masterfrom

kou:travis-fix-bundler-error

Commits on May 6, 2016

Fix Bundler related error on Travis CI
kou
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Commits on May 7, 2016

Use json 1.8.3 because Ruby 2.3 or later bundles json 1.8.3
kou
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