Bump gson from 2.8.7 to 2.8.8

Bumps [gson](https://github.com/google/gson) from 2.8.7 to 2.8.8.
- [Release notes](https://github.com/google/gson/releases)
- [Changelog](https://github.com/google/gson/blob/master/CHANGELOG.md)
- [Commits](google/gson@gson-parent-2.8.7...gson-parent-2.8.8)

---
updated-dependencies:
- dependency-name: com.google.code.gson:gson
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

dependencies.gradle

@@ -55,7 +55,7 @@ final Map<String, String> libraries = [
   freemarker          : 'org.freemarker:freemarker:2.3.31',
   gradleDownload      : 'de.undercouch:gradle-download-task:4.1.2',
   grolifant           : 'org.ysb33r.gradle:grolifant:0.17.0',
-  gson                : 'com.google.code.gson:gson:2.8.7',
+  gson                : 'com.google.code.gson:gson:2.8.8',
   guava               : 'com.google.guava:guava:30.1.1-jre',
   h2                  : 'com.h2database:h2:1.4.200',
   hamcrest            : 'org.hamcrest:hamcrest-core:2.2',