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.
Since we support typed property, this can be simply
```java
clientConfig.get(CommonClientConfigKey.ServerListRefreshInterval, LISTOFSERVERS_CACHE_REPEAT_INTERVAL);
```