Gradle: Use tested version of Mockito

The Ant JUnit tests have been tested previously with 2.5.0 as working.
2.11.0 is causing issues that are manifesting as Gradle-specific test
failures. We can upgrade this explicitly when we also upgrade the
dependency in the Ant-based CI system.

build.gradle

@@ -101,7 +101,7 @@ configure(javaProjects) {
     dependencies {
         testCompile 'junit:junit:4.+'
         testCompile 'org.hamcrest:hamcrest-library:1.3'
-        testCompile 'org.mockito:mockito-core:2.11.0'
+        testCompile 'org.mockito:mockito-core:2.5.0'
     }
 
     sourceCompatibility = 1.7