Travis build - replace oraclejdk8 with openjdk8

Recent builds (e.g. https://travis-ci.org/scoverage/scalac-scoverage-plugin/builds/545010217) failed with 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"
install-jdk.sh 2019-05-02
Expected feature release number in range of 9 to 13, 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

@@ -4,7 +4,7 @@ script:
   - sbt ++$TRAVIS_SCALA_VERSION test
 
 jdk:
-  - oraclejdk8
+  - openjdk8
 
 scala:
   - 2.10.7