Make project compile under Java 8.

Most of the errors were coming from stricter rules for javadoc. Findbugs was upgraded
to version 3.0.0, and PowerMock to 1.5.4 to have Java 8 support.
Additionally gradle wrapper was updated to the latest 1.x version.
Same as above, this can be written as

```java
serverWeightTaskTimerInterval = clientConfig.get(WEIGHT_TASK_TIMER_INTERVAL_CONFIG_KEY,  DEFAULT_TIMER_INTERVAL);
```