support travis build

[yuokada]

see these repositories

• https://github.com/embulk/embulk-input-s3
• https://github.com/muga/embulk-filter-calcite
• https://github.com/sonots/embulk-filter-column/blob/master/.travis.yml
• https://github.com/takumakanari/embulk-input-http