refine travis-ci scripts
diff --git a/.travis.yml b/.travis.yml
index 845e35d..ae87331 100755
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,7 +15,9 @@
 
 language: java
 sudo: false
-
+cache:
+  directories:
+    - $HOME/.m2
 jdk:
   - openjdk8
   - openjdk11