Configure travis to use the apt add-on to add libraries in ubuntu (in…

…stead of manually calling apt-get)

.travis.yml

@@ -1,7 +1,9 @@
 language: java
 sudo: false
 dist: xenial
-before_install:
-  - sudo apt-get -qq update
-  - sudo apt-get install -y libvoikko1 voikko-fi
+addons:
+  apt:
+    packages:
+     - libvoikko1
+     - voikko-fi
 install: mvn install -DskipTests=true -Dmaven.javadoc.skip=true -B -V -Dgpg.skip=true