try using a build matrix for Travis

.travis.yml

@@ -1,13 +1,31 @@
 # https://travis-ci.org/jwtk/jjwt
+
+dist: trusty
+sudo: required
+
 language: java
 
-jdk:
-  - oraclejdk7
-  - oraclejdk8
+matrix:
+  include:
+    - jdk: oraclejdk8
+      addons:
+      apt:
+        packages:
+          - oracle-java8-installer
+  include:
+    - jdk: oraclejdk7
+      addons:
+      apt:
+        packages:
+          - oracle-java7-installer
 
 before_install:
   - export BUILD_COVERAGE="$([ $TRAVIS_JDK_VERSION == 'oraclejdk8' ] && echo 'true')"
+
 install: echo "No need to run mvn install -DskipTests then mvn install.  Running mvn install."
+
 script: mvn install
+
 after_success:
   - test -z "$BUILD_COVERAGE" || mvn clean test jacoco:report coveralls:report
+