Fix mockito-core version

This is an attempt to prevent following error from the Maven Central Repository:

Invalid POM: /io/tus/java/client/tus-java-client/0.4.2/tus-java-client-0.4.2.pom: Invalid version for Dependency {groupId=org.mockito, artifactId=mockito-core, version=2.+, type=jar} - uses invalid dynamic version syntax. Replace with a fixed version or standard mathematical notation e.g., [1.5,) for version 1.5 and higher. Dropping existing partial staging repository.

build.gradle

@@ -18,7 +18,7 @@ dependencies {
     compile 'org.jetbrains:annotations:13.0'
     testCompile 'junit:junit:4.12'
     testCompile 'org.mock-server:mockserver-netty:5.2.2'
-    testCompile 'org.mockito:mockito-core:2.+'
+    testCompile 'org.mockito:mockito-core:2.28.2'
 }
 
 task sourcesJar(type: Jar, dependsOn: classes) {