travis: update go versions
travis: update go versions

As discussed at https://github.com/Shopify/sarama/pull/950.
VividCortex/sarama,remerge/sarama,VividCortex/sarama,mailgun/sarama,Shopify/sarama,vrischmann/sarama,DataDog/sarama,mailgun/sarama,DataDog/sarama,vrischmann/sarama,Shopify/sarama,remerge/sarama