Gradle: Upload signed artifacts to Sonatype OSS Nexus for Maven Central

http://yennicktrevels.com/blog/2013/10/11/automated-gradle-project-deployment-to-sonatype-oss-repository/

Using a 'spongycastle' subgroup (ie com.madgag.spongycastle) to give a useful
grouping, apparently this is legit:

https://docs.sonatype.org/display/Repository/Choosing+your+Coordinates

Also guard against missing gradle.properties for travis, which doesn't have
to do signing or uploading to sonatype.