Update CI Logstash version matrix (#17)

.travis.yml

@@ -6,9 +6,11 @@ matrix:
   - rvm: jruby-9.1.13.0
     env: LOGSTASH_BRANCH=master
   - rvm: jruby-9.1.13.0
-    env: LOGSTASH_BRANCH=6.x
+    env: LOGSTASH_BRANCH=7.0
   - rvm: jruby-9.1.13.0
-    env: LOGSTASH_BRANCH=6.5
+    env: LOGSTASH_BRANCH=6.7
+  - rvm: jruby-9.1.13.0
+    env: LOGSTASH_BRANCH=6.6
   fast_finish: true
 install: true
 script: ci/build.sh