cache maven dir

.travis.yml

@@ -3,6 +3,9 @@ sudo: required
 dist: trusty
 jdk:
 - oraclejdk8
+cache:
+  directories:
+  - $HOME/.m2
 env:
   global:
     - secure: "IfYURwZaDWuBDvyn47n0k1Zod/IQw1FF+CS5nnV08Q+NfC3vGGJMwV8m59XnbfwnWGxwvCaAbk4qP6s6+ijgZNKkvgfFMo3rfTok5zt43bIqgaFOANYV+OC/1c59gYD6ZUxhW5iNgMgU3qdsRtJuwSmfkVv/jKyLGfAbS4kN8BA=" #coverity