Bump com.google.guava:guava from 18.0 to 23.0

Bumps [com.google.guava:guava](https://github.com/google/guava) from 18.0 to 23.0.
- [Release notes](https://github.com/google/guava/releases)
- [Commits](google/guava@v18.0...v23.0)

---
updated-dependencies:
- dependency-name: com.google.guava:guava
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

core/build.gradle

@@ -13,7 +13,7 @@ compileJava {
 dependencies {
     api fileTree(dir: 'libs', include: ['*.jar'])
 
-    api 'com.google.guava:guava:20.0' //TODO ToStringHelper update for updating to ?
+    api 'com.google.guava:guava:23.0' //TODO ToStringHelper update for updating to ?
     api 'com.google.code.findbugs:jsr305:3.0.2'
     api 'com.madgag.spongycastle:core:1.58.0.0'
     api 'com.lambdaworks:scrypt:1.4.0'