Fix CI: upgrade TravisCI from JDK 7 to JDK 8
Fix CI: upgrade TravisCI from JDK 7 to JDK 8

Indeed, TravisCI's minimum Java version now is v8.
See also: https://docs.travis-ci.com/user/reference/xenial/#jvm-clojure-groovy-java-scala-support

Previously, build would be failing with:

```
Installing openjdk7
$ export JAVA_HOME=~/openjdk7
$ export PATH="$JAVA_HOME/bin:$PATH"
$ ~/bin/install-jdk.sh --target "/home/travis/openjdk7" --workspace "/home/travis/.cache/install-jdk" --feature "7" --license "GPL" --cacerts
Ignoring license option: GPL -- using GPLv2+CE by default
install-jdk.sh 2020-06-02
Expected feature release number in range of 9 to 16, but got: 7
The command "~/bin/install-jdk.sh --target "/home/travis/openjdk7" --workspace "/home/travis/.cache/install-jdk" --feature "7" --license "GPL" --cacerts" failed and exited with 3 during .
Your build has been stopped.
```

Source: https://travis-ci.org/github/marccarre/properties-files-maven-plugin/jobs/699498222

marccarre/properties-files-maven-plugin