Travis configuration - upgrade JDK version from 1.7 to 1.8

Change "oraclejdk7" to "oraclejdk8" because "oraclejdk7" is not supported by Travis anymore. See job log fragment:

$ jdk_switcher use oraclejdk7
Switching to Oracle JDK7 (java-7-oracle), JAVA_HOME will be set to /usr/lib/jvm/java-7-oracle
update-java-alternatives: directory does not exist: /usr/lib/jvm/java-7-oracle
...
$ java -Xmx32m -version
Picked up _JAVA_OPTIONS: -Xmx2048m -Xms512m
java version "1.8.0_144"
Java(TM) SE Runtime Environment (build 1.8.0_144-b01)
Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)

Author: gslowikowski

.travis.yml

@@ -10,4 +10,4 @@ install:
 
 script: mvn verify --update-snapshots --settings .travis.settings.xml -e
 
-jdk: oraclejdk7
+jdk: oraclejdk8