Migrate to go modules. The `go.mod` file is not committed into the repository and contains entries for dependencies exclusively needed by the repository and none of the integrations. This is so because we want the importing program to have the final word in deciding versions. As a side-effect, we were forced to update the import path of what was formerly `gopkg.in/Shopify/sarama.v1` to `github.com/Shopify/sarama`. For more details, please see: https://github.com/Shopify/sarama/issues/1510

Additionally, CI change were introduced which reduce time by about 50%.