[skip ci] Travis: update LOGSTASH_BRANCH from 6.[0..4] to 6.5

Author: robbavey

.travis.yml

@@ -9,7 +9,7 @@ matrix:
  - rvm: jruby-9.1.13.0
  env: LOGSTASH_BRANCH=6.x
  - rvm: jruby-9.1.13.0
- env: LOGSTASH_BRANCH=6.0
+ env: LOGSTASH_BRANCH=6.5
  - rvm: jruby-1.7.27
  env: LOGSTASH_BRANCH=5.6
  fast_finish: true