Travis build - replace oraclejdk8 with openjdk8

Recent build https://travis-ci.org/scoverage/scoverage-maven-plugin/builds/576250498 failed with an error:

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"
Ignoring license option: BCL -- using GPLv2+CE by default
install-jdk.sh 2019-07-17
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 .

Author: gslowikowski

.travis.yml

@@ -10,4 +10,4 @@ install:
 
 script: mvn verify animal-sniffer:check --update-snapshots --settings .travis.settings.xml -e
 
-jdk: oraclejdk8
+jdk: openjdk8