remove jdk6 from travis options for our java 7 supported branches
remove jdk6 from travis options for our java 7 supported branches

fubuki/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,vorce/es-metrics,aparo/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,vorce/es-metrics,aparo/elasticsearch,vorce/es-metrics,aparo/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,vorce/es-metrics,vorce/es-metrics