Update Travis CI to test using JDK8

Update the JDK versions Travis uses for CI for aws-dynamodb-encryption-java.
Today, these tests fail because the minimum supported version is JDK7.

Author: johnwalker

.travis.yml

@@ -1,7 +1,7 @@
 language: java
 jdk:
-  - openjdk7
-  - oraclejdk7
+  - openjdk8
+  - oraclejdk8
 install: /bin/true
 script: mvn install --quiet -Dgpg.skip=true -DskipTests=true