Specify cache for "Travis".

Cf. https://markmail.org/message/vhwelfs2qypyvo4l
diff --git a/.travis.yml b/.travis.yml
index 116a2ba..e6d7d4d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,6 +16,10 @@
 language: java
 sudo: false
 
+cache:
+  directories:
+    - "$HOME/.m2"
+
 jdk:
   - openjdk8