Download dependencies with JDK 8

As of June 18th 2018, Maven Central supports TLS 1.2 only. Therefore we
cannot install dependencies with JDK 6 anymore. As a workaround we
switch in each build temporarily to JDK 8 for downloading the
dependencies. Afterwards we restore the desired JDK and build System
Rules with it.

.travis.yml

@@ -1,5 +1,16 @@
 language: java
-install: ./mvnw install -DskipTests=true -Dgpg.skip=true
+install:
+  # Download dependencies with JDK 8 because Mave Central supports 
+  # TLS 1.2 only but OpenJDK 6 does not.
+  - export ORIGINAL_JAVA_HOME=$JAVA_HOME
+  - jdk_switcher use oraclejdk8     
+  - ./mvnw test -DskipTests
+  # Delete all files created with JDK 8
+  - ./mvnw clean
+  # Restore desired JDK
+  - export JAVA_HOME=$ORIGINAL_JAVA_HOME
+  # Run original install command but without GPG signing
+  - ./mvnw install -DskipTests=true -Dgpg.skip=true
 addons:
   apt:
     packages: