Make scala version an env var and clean up hard coded kafka version references
Make scala version an env var and clean up hard coded kafka version references

tinglinux/logstash-kafka,Aloomaio/logstash-kafka,opentable/logstash-kafka,tinglinux/logstash-kafka,joekiller/logstash-kafka,joekiller/logstash-kafka,Aloomaio/logstash-kafka,opentable/logstash-kafka