update travis configuration

Travis allows many concurrent builds and is easier to
work with for pull requests at the moment over Circle.
`mvn test` runs in half the time (it only runs the
tests once), and it can be easily configured to build on
pushes, not just the tip of branches.

1. use java defaults, just `mvn test`
2. use travis container infrastructure via `sudo: false`
3. test again JDK8

.travis.yml

@@ -1,2 +1,4 @@
-language: clojure
-script: lein test
+language: java
+jdk:
+  - oraclejdk8
+sudo: false