Bump assertj-core from 3.12.2 to 3.13.2

Bumps [assertj-core](https://github.com/joel-costigliola/assertj-core) from 3.12.2 to 3.13.2.
- [Release notes](https://github.com/joel-costigliola/assertj-core/releases)
- [Commits](assertj/assertj@assertj-core-3.12.2...assertj-core-3.13.2)

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

build.gradle

@@ -70,7 +70,7 @@ dependencies {
 
     testCompile group: 'junit', name: 'junit', version: '4.12'
     testCompile group: 'com.github.stefanbirkner', name: 'system-rules', version: '1.19.0'
-    testCompile group: 'org.assertj', name: 'assertj-core', version: '3.12.2'
+    testCompile group: 'org.assertj', name: 'assertj-core', version: '3.13.2'
     testCompile group: 'org.mockito', name: 'mockito-core', version: '3.0.0'
     testCompile group: 'org.skyscreamer', name: 'jsonassert', version: '1.5.0'
     testCompile group: 'org.jsoup', name: 'jsoup', version: '1.12.1'