Specify openjdk8 instead of oraclejdk8 as the Travis CI build fai…

…ls as follows otherwise:

```
Installing oraclejdk8
$ export JAVA_HOME=~/oraclejdk8
$ export PATH="$JAVA_HOME/bin:$PATH"
$ ~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"
install-jdk.sh 2019-05-02
Expected feature release number in range of 9 to 14, but got: 8
The command "~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"" failed and exited with 3 during .
Your build has been stopped.
```

Signed-off-by: Robin Verduijn <robinverduijn.github@gmail.com>

.travis.yml

@@ -1,7 +1,7 @@
 language: groovy
 
 jdk:
-  - oraclejdk8
+  - openjdk8
 
 # Relax CI memory constraints by not running in containers.
 sudo: required