ci: speed up builds by enabling caching for ~/.m2 directory.

See https://docs.travis-ci.com/user/caching/#arbitrary-directories

Author: php-coder

.travis.yml

@@ -68,6 +68,10 @@ deploy:
 jdk:
   - openjdk8
 
+cache:
+  directories:
+  - $HOME/.m2
+
 services:
   - mysql