Realm Unit Test on Android

Trying to unit test a class that does some calls to Realm (0.87.4), the test setup fails with

java.lang.NoClassDefFoundError: rx/Observable$OnSubscribe
at io.realm.RealmConfiguration$Builder.<init>(RealmConfiguration.java:279)
at org.testapp.db.MyClassTest.setUp(MyClassTest.java:34)
... 
Caused by: java.lang.ClassNotFoundException: rx.Observable$OnSubscribe
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)

My test class starts with:

@RunWith(MockitoJUnitRunner.class)
public class MyClassTest extends TestCase {

@Rule
public TemporaryFolder testFolder = new TemporaryFolder();

Realm realm;

@Before
public void setUp() throws Exception {
    File tempFolder = testFolder.newFolder("realmdata");
    RealmConfiguration config = new RealmConfiguration.Builder(tempFolder).build();

    realm = Realm.getInstance(config);
}
...

My gradle has:

testCompile 'junit:junit:4.12'
testCompile "org.mockito:mockito-core:1.10.19"
testCompile "org.robolectric:robolectric:3.0"
compile 'io.realm:realm-android:0.87.4'

How to solve this?