refine travis-ci scripts
diff --git a/.travis.yml b/.travis.yml
index 88f36fb..6ed1e64 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,12 +15,16 @@
 

 language: java

 sudo: false

-

-jdk:

-  - oraclejdk11

-  - openjdk8

-  - openjdk11

-  - openjdk12

-  - openjdk13

-  - openjdk-ea

-

+cache:

+  directories:

+    - $HOME/.m2

+matrix:

+  include:

+    - jdk: openjdk8

+    - jdk: openjdk11

+    - jdk: openjdk12

+    - jdk: openjdk13

+    - jdk: oraclejdk11

+    - jdk: openjdk-ea

+  allow_failures:

+    - jdk: openjdk-ea