Adding travis_wait to mvn install

Why ?
Building such big APIs as Adyen APIs   can take a lot of time.  It's possible to get a time out.
Here (4hwc/4HWCAutonomousCar@96c6396), I got a time out due to generating javadoc. Then, I decided to add travis_wait and it works pretty well.

Official docs : https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received

.travis.yml

@@ -6,7 +6,8 @@ jdk:
   - oraclejdk8
   - openjdk7
 
-install: mvn install -Dgpg.skip=true
+#travis_wait : avoid time out
+install: travis_wait mvn install -Dgpg.skip=true
 
 sudo: false