Publish WWSK maven artifacts to JCenter

[emxsys]

WWSK maven artifacts in Bintray (https://bintray.com/nasaworldwind/maven/WorldWindServerKit) should be published to JCenter.

[pdavidc]

We can publish WWSK maven artifacts to JCenter once our Bintray premium trial expires. See NASAWorldWind/NASAWorldWind.github.io#36 for status.