Revert version to 0.4.15
Revert version to 0.4.15

cosmo0920/fluent-plugin-google-cloud,a-robinson/fluent-plugin-google-cloud,GoogleCloudPlatform/fluent-plugin-google-cloud,laramiel/fluent-plugin-google-cloud,arthurdarcet/fluent-plugin-google-cloud,senthilkumar-kj/fluent-plugin-google-cloud