Travis - add 6.0 build, remove default JRuby 1.7 build, bump RVM versions

Author: jakelandis

.travis.yml

@@ -2,18 +2,17 @@
 sudo: false
 language: ruby
 cache: bundler
-env: 
-rvm:
-- jruby-1.7.25
 matrix:
   include:
-  - rvm: jruby-9.1.10.0
+  - rvm: jruby-9.1.13.0
     env: LOGSTASH_BRANCH=master
-  - rvm: jruby-9.1.10.0
+  - rvm: jruby-9.1.13.0
     env: LOGSTASH_BRANCH=6.x
-  - rvm: jruby-1.7.25
+  - rvm: jruby-9.1.13.0
+    env: LOGSTASH_BRANCH=6.0
+  - rvm: jruby-1.7.27
     env: LOGSTASH_BRANCH=5.6
   fast_finish: true
 install: true
 script: ci/build.sh
-jdk: oraclejdk8
+jdk: oraclejdk8