Bump opencensus-stackdriver to 0.3.1. (#237)
Bump opencensus-stackdriver to 0.3.1. (#237)

This matches change in output plugin here: https://github.com/GoogleCloudPlatform/fluent-plugin-google-cloud/pull/369
GoogleCloudPlatform/google-fluentd,GoogleCloudPlatform/google-fluentd,GoogleCloudPlatform/google-fluentd