Use OpenJDK instead of Oracle.

On Travis, the Oracle one no longer exists.

.travis.yml

@@ -3,7 +3,7 @@ scala:
   - 2.13.0
   - 2.12.8
   - 2.11.12
-jdk: oraclejdk8
+jdk: openjdk8
 script:
   - sbt clean coverage 'testOnly -- showtimes true timeFactor 5 exclude no-ci'
   - if [[ -n "$CODACY_PROJECT_TOKEN" ]]; then sbt coverageReport coverageAggregate codacyCoverage; else echo "Skipped coverage reporting since no token was found"; fi