Update dependencies

[cosmo0920]

• Depends on fluent-plugin-elasticsearch plugin v4.0.0
• Use newer bundler and rake

[cosmo0920]

I'd published fluent-plugin-elasticsearch v4.0.0 which contains breaking changes for ILM feature but this version's breaking changes does not affect your plugin.
Your plugin uses fluent-plugin-elasticsearch dependencies:[">= 3.3.0", "< 4"] but this dependency should be [">= 3.3.0", "< 5"] now.
@atomita Could you release the new version?

[cosmo0920]

@atomita Kindly ping.