Update to v3.2.0

[simukappu]

Description of changes:

3.2.0

• Feature - Add placeholder support for stream names to send records to multiple streams : #165 #174
• Enhancement - Add sts_endpoint_url configuration parameter to support AWS STS regional endpoints : #186
• Enhancement - Add aws_ses_token configuration parameter to use IAM with MFA and directly provided temporary credentials : #166
• Dependency - Update gem dependency to Ruby 2.3.0+ and Fluentd 0.14.22+
• Bug - Fix dependency problem on AWS SDK with td-agent v3.4.1

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