Bump gson from 2.8.8 to 2.8.9

Bumps [gson](https://github.com/google/gson) from 2.8.8 to 2.8.9.
- [Release notes](https://github.com/google/gson/releases)
- [Changelog](https://github.com/google/gson/blob/master/CHANGELOG.md)
- [Commits](google/gson@gson-parent-2.8.8...gson-parent-2.8.9)

---
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>

Author: dependabot[bot]

app/build.gradle

@@ -109,7 +109,7 @@ dependencies {
     implementation 'com.squareup.retrofit2:retrofit:2.6.4'
 
     // Gson
-    implementation 'com.google.code.gson:gson:2.8.8'
+    implementation 'com.google.code.gson:gson:2.8.9'
 
     // ReactiveX
     implementation 'io.reactivex.rxjava2:rxjava:2.2.21'