Permit use of @JsonTest without Jackson

[wonwoo]

I think @jsontest is not only Jackson.

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
    </dependency>
</dependencies>

No dependency Jackson.

@RunWith(SpringRunner.class)
@JsonTest
public class SpringBootJsontestApplicationTests {

    @Autowired
    private GsonTester<VehicleDetails> json;
    ...
}

Error Message
java.lang.NoClassDefFoundError: com/fasterxml/jackson/databind/Module

[snicoll]

@wonwoo good catch, thank you very much for the PR. This is now merged as of 1.5.x all the way down to master.